[xDS] start splitting up `grpc_xds_client` BUILD target (#37032)

This moves some low-hanging fruit out of the `grpc_xds_client` BUILD target:
- `xds_certificate_provider`
- `xds_certificate_provider_store`
- `xds_credentials`
- `xds_file_watcher_certificate_provider_factory`

Subsequent PRs will do more refactoring.

Closes #37032

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37032 from markdroth:xds_build_target_refactoring eac85eb7bc
PiperOrigin-RevId: 646627448

src/core/BUILD

@@ -5076,14 +5076,139 @@ grpc_cc_library(
 )
 
 grpc_cc_library(
-    name = "grpc_xds_client",
+    name = "xds_certificate_provider",
+    srcs = [
+        "xds/grpc/xds_certificate_provider.cc",
+    ],
+    hdrs = [
+        "xds/grpc/xds_certificate_provider.h",
+    ],
+    external_deps = [
+        "absl/base:core_headers",
+        "absl/functional:bind_front",
+        "absl/log:check",
+        "absl/log:log",
+        "absl/strings",
+        "absl/types:optional",
+    ],
+    language = "c++",
+    tags = ["nofixdeps"],
+    deps = [
+        "channel_args",
+        "error",
+        "grpc_matchers",
+        "grpc_tls_credentials",
+        "unique_type_name",
+        "useful",
+        "//:gpr",
+        "//:grpc_base",
+        "//:ref_counted_ptr",
+        "//:tsi_ssl_credentials",
+    ],
+)
+
+grpc_cc_library(
+    name = "xds_certificate_provider_store",
     srcs = [
-        "lib/security/credentials/xds/xds_credentials.cc",
         "xds/grpc/certificate_provider_store.cc",
+    ],
+    hdrs = [
+        "xds/grpc/certificate_provider_store.h",
+    ],
+    external_deps = [
+        "absl/base:core_headers",
+        "absl/log:log",
+        "absl/strings",
+    ],
+    language = "c++",
+    tags = ["nofixdeps"],
+    deps = [
+        "certificate_provider_factory",
+        "certificate_provider_registry",
+        "grpc_tls_credentials",
+        "json",
+        "json_args",
+        "json_object_loader",
+        "unique_type_name",
+        "useful",
+        "validation_errors",
+        "//:config",
+        "//:gpr",
+        "//:grpc_base",
+        "//:orphanable",
+        "//:ref_counted_ptr",
+    ],
+)
+
+grpc_cc_library(
+    name = "xds_credentials",
+    srcs = [
+        "lib/security/credentials/xds/xds_credentials.cc",
+    ],
+    hdrs = [
+        "lib/security/credentials/xds/xds_credentials.h",
+    ],
+    external_deps = [
+        "absl/status",
+        "absl/log:check",
+        "absl/log:log",
+        "absl/types:optional",
+    ],
+    language = "c++",
+    tags = ["nofixdeps"],
+    deps = [
+        "channel_args",
+        "grpc_lb_xds_channel_args",
+        "grpc_matchers",
+        "grpc_tls_credentials",
+        "unique_type_name",
+        "useful",
+        "xds_certificate_provider",
+        "//:channel_arg_names",
+        "//:gpr",
+        "//:grpc_base",
+        "//:grpc_core_credentials_header",
+        "//:grpc_credentials_util",
+        "//:grpc_security_base",
+        "//:ref_counted_ptr",
+    ],
+)
+
+grpc_cc_library(
+    name = "xds_file_watcher_certificate_provider_factory",
+    srcs = [
         "xds/grpc/file_watcher_certificate_provider_factory.cc",
+    ],
+    hdrs = [
+        "xds/grpc/file_watcher_certificate_provider_factory.h",
+    ],
+    external_deps = [
+        "absl/log:log",
+        "absl/strings",
+        "absl/strings:str_format",
+    ],
+    language = "c++",
+    tags = ["nofixdeps"],
+    deps = [
+        "certificate_provider_factory",
+        "grpc_tls_credentials",
+        "json",
+        "json_args",
+        "json_object_loader",
+        "time",
+        "validation_errors",
+        "//:config",
+        "//:gpr",
+        "//:grpc_base",
+        "//:ref_counted_ptr",
+    ],
+)
+
+grpc_cc_library(
+    name = "grpc_xds_client",
+    srcs = [
         "xds/grpc/xds_audit_logger_registry.cc",
         "xds/grpc/xds_bootstrap_grpc.cc",
-        "xds/grpc/xds_certificate_provider.cc",
         "xds/grpc/xds_client_grpc.cc",
         "xds/grpc/xds_cluster.cc",
         "xds/grpc/xds_cluster_specifier_plugin.cc",
@@ -5101,12 +5226,8 @@ grpc_cc_library(
         "xds/grpc/xds_transport_grpc.cc",
     ],
     hdrs = [
-        "lib/security/credentials/xds/xds_credentials.h",
-        "xds/grpc/certificate_provider_store.h",
-        "xds/grpc/file_watcher_certificate_provider_factory.h",
         "xds/grpc/xds_audit_logger_registry.h",
         "xds/grpc/xds_bootstrap_grpc.h",
-        "xds/grpc/xds_certificate_provider.h",
         "xds/grpc/xds_client_grpc.h",
         "xds/grpc/xds_cluster.h",
         "xds/grpc/xds_cluster_specifier_plugin.h",
@@ -5251,6 +5372,10 @@ grpc_cc_library(
         "upb_utils",
         "useful",
         "validation_errors",
+        "xds_certificate_provider",
+        "xds_certificate_provider_store",
+        "xds_credentials",
+        "xds_file_watcher_certificate_provider_factory",
         "xds_type_upb",
         "xds_type_upbdefs",
         "//:channel",
@@ -5339,6 +5464,9 @@ grpc_cc_library(
         "resolved_address",
         "slice_refcount",
         "unique_type_name",
+        "xds_certificate_provider",
+        "xds_certificate_provider_store",
+        "xds_credentials",
         "//:api_trace",
         "//:config",
         "//:debug_location",
@@ -5483,6 +5611,7 @@ grpc_cc_library(
         "resolved_address",
         "subchannel_interface",
         "validation_errors",
+        "xds_credentials",
         "xds_dependency_manager",
         "//:call_tracer",
         "//:config",