[ObjC] generate Package.swift from template (#33505)

This is to help simplify the grpc iOS release flow.

Package.swift was manually written and out of sync for a long time in
this repo.
They are currently being updated updated for releases in
[grpc-ios](https://github.com/grpc/grpc-ios) repo, which includes a lot
of manual work to include new files and exclude tests.

This diff generate Package.swift from a template like other languages to
avoid the tedious work. Currently only gRPC-Core is generated form the
template, gRPC-cpp remains the same for now as they don't change often.

CC @sampajano

build_handwritten.yaml

@@ -202,3 +202,10 @@ ruby_gem:
   - boringssl
   - re2
   - z
+swift_package:
+  deps:
+  - grpc
+  - grpc_authorization_provider
+  - gpr
+  - upb
+  - re2

templates/Package.swift.template

@@ -0,0 +1,121 @@
+%YAML 1.2
+--- |
+    // swift-tools-version:5.5
+    // The swift-tools-version declares the minimum version of Swift required to build this package.
+    import PackageDescription
+    import Foundation
+
+    var basePath = FileManager.default.fileExists(atPath: "native") ? "native" : "."
+    
+    let package = Package(
+      name: "gRPC",
+      products: [
+        .library(
+          name: "gRPC-Core",
+          targets: [
+            "gRPC-Core",
+          ]
+        ),
+        .library(
+          name: "gRPC-cpp",
+          targets: [
+            "gRPC-cpp",
+          ]
+        )
+      ],
+    
+      dependencies: [
+        .package(url: "https://github.com/firebase/abseil-cpp-SwiftPM.git", "0.20220623.0"..<"0.20220624.0"),
+        .package(url: "https://github.com/firebase/boringssl-SwiftPM.git", "0.9.0"..<"0.10.0"),
+      ],
+    
+      targets: [
+        .target(
+          name: "gRPC-Core",
+          dependencies: [
+            .product(name:"abseil", package: "abseil-cpp-SwiftPM"),
+            .product(name:"openssl_grpc", package: "boringssl-SwiftPM"),
+          ],
+          path: basePath,
+          exclude: [
+            "examples/",
+            "src/objective-c/",
+          ],
+        <%
+          files = []
+          lib_maps = {lib.name: lib for lib in libs}
+          for dep in swift_package.get('deps', []):
+            lib = lib_maps[dep]
+            files.extend(lib.get('public_headers', []) + lib.headers + lib.src)
+          files = sorted(set(files))
+        %>
+          sources: [
+            % for file in files:
+            "${file}",
+            % endfor
+          ],
+          publicHeadersPath: "spm-core-include",
+          cSettings: [
+            .headerSearchPath("./"),
+            .headerSearchPath("include/"),
+            .headerSearchPath("third_party/re2/"),
+            .headerSearchPath("third_party/upb/"),
+            .headerSearchPath("third_party/utf8_range/"),
+            .headerSearchPath("third_party/xxhash/"),
+            .headerSearchPath("src/core/ext/upb-generated/"),
+            .headerSearchPath("src/core/ext/upbdefs-generated/"),
+            .define("GRPC_ARES", to: "0"),
+          ],
+          linkerSettings: [
+            .linkedFramework("CoreFoundation"),
+            .linkedLibrary("z"),
+          ]
+        ),
+        .target(
+          name: "gRPC-cpp",
+          dependencies: [
+            .product(name:"abseil", package: "abseil-cpp-SwiftPM"),
+            "gRPC-Core",
+          ],
+          path: basePath,
+          exclude: [
+            "examples/",
+            "src/cpp/client/cronet_credentials.cc",
+            "src/cpp/client/channel_test_peer.cc",
+            "src/cpp/common/alts_util.cc",
+            "src/cpp/common/alts_context.cc",
+            "src/cpp/common/insecure_create_auth_context.cc",
+            "src/cpp/server/admin/",
+            "src/cpp/server/channelz/",
+            "src/cpp/server/csds/",
+            "src/cpp/server/load_reporter/",
+            "src/cpp/ext/",
+            "src/cpp/util/core_stats.cc",
+            "src/cpp/util/core_stats.h",
+            "src/cpp/util/error_details.cc",
+            "src/objective-c/examples/",
+            "src/objective-c/manual_tests/",
+            "src/objective-c/tests/",
+          ],
+          sources: [
+            "src/cpp/",
+          ],
+          publicHeadersPath: "spm-cpp-include",
+          cSettings: [
+            .headerSearchPath("./"),
+            .headerSearchPath("include/"),
+            .headerSearchPath("third_party/upb/"),
+            .headerSearchPath("src/core/ext/upb-generated"),
+          ]
+        ),
+        .testTarget(
+          name: "build-test",
+          dependencies: [
+            "gRPC-cpp",
+          ],
+          path: basePath + "/test/spm_build"
+        ),
+      ],
+      cLanguageStandard: .gnu11,
+      cxxLanguageStandard: .cxx14
+    )