api: xDS API version specifier (#9468)

It enables to specify API versioning for xDS for #8421, allow to write like this config

eds_cluster_config:
    eds_config:
       version: V2
       api_config_source:
          api_type: GRPC
	  grpc_services:
            envoy_grpc:
               cluster_name: eds_cluster

Risk Level: Low
Testing: unit test(maybe it is needed to add integration test)

Signed-off-by: shikugawa <rei@tetrate.io>

Mirrored from https://github.com/envoyproxy/envoy @ e3717b54b8d91d1862b096f73efbe96086862183

envoy/api/v2/core/config_source.proto

@@ -103,8 +103,20 @@ message RateLimitSettings {
 // <arch_overview_service_discovery>` etc. may either be sourced from the
 // filesystem or from an xDS API source. Filesystem configs are watched with
 // inotify for updates.
-// [#next-free-field: 6]
+// [#next-free-field: 7]
 message ConfigSource {
+  enum XdsApiVersion {
+    // use for describing explicitly that xDS API version is set automatically. In default, xDS API
+    // version is V2
+    AUTO = 0;
+
+    // use xDS v2 API
+    V2 = 1;
+
+    // use xDS v3alpha API
+    V3ALPHA = 2;
+  }
+
   oneof config_source_specifier {
     option (validate.required) = true;
 
@@ -150,4 +162,7 @@ message ConfigSource {
   // means no timeout - Envoy will wait indefinitely for the first xDS config (unless another
   // timeout applies). The default is 15s.
   google.protobuf.Duration initial_fetch_timeout = 4;
+
+  // API version for xDS endpoint
+  XdsApiVersion xds_api_version = 6;
 }

envoy/api/v3alpha/core/config_source.proto

@@ -113,10 +113,22 @@ message RateLimitSettings {
 // <arch_overview_service_discovery>` etc. may either be sourced from the
 // filesystem or from an xDS API source. Filesystem configs are watched with
 // inotify for updates.
-// [#next-free-field: 6]
+// [#next-free-field: 7]
 message ConfigSource {
   option (udpa.annotations.versioning).previous_message_type = "envoy.api.v2.core.ConfigSource";
 
+  enum XdsApiVersion {
+    // use for describing explicitly that xDS API version is set automatically. In default, xDS API
+    // version is V2
+    AUTO = 0;
+
+    // use xDS v2 API
+    V2 = 1;
+
+    // use xDS v3alpha API
+    V3ALPHA = 2;
+  }
+
   oneof config_source_specifier {
     option (validate.required) = true;
 
@@ -162,4 +174,7 @@ message ConfigSource {
   // means no timeout - Envoy will wait indefinitely for the first xDS config (unless another
   // timeout applies). The default is 15s.
   google.protobuf.Duration initial_fetch_timeout = 4;
+
+  // API version for xDS endpoint
+  XdsApiVersion xds_api_version = 6;
 }