Add xDS resource services.

[minwoox]

Motivation: We need APIs for creating, updating and deleting xDS resources.

Modifications:

• Added xds_{resource}.proto files that defines APIs for xDS resources.
  • The API provides both gRPC and HTTP API.
  • The API follows the API Improvement Proposals by Google so it's recommnded to read the documentation especially Create, Update and Delete.
• Added XdsClusterService, XdsEndpointService, XdsListenerService and XdsRouteService.
  • They implemented the Xds{Resource}ServiceImplBase generated from the proto files.
  • Each create API request has parent (groups/{group}) and {resource}_id.
  • The resource name is automatically composed using those fields: groups/{group}/{resources}/{resource}_id. e.g groups/foo/clusters/echo-cluster
  • Thus, the resource name in the resource is ignored: https://github.com/aip-dev/google.aip.dev/blob/master/aip/general/0133.md#user-specified-ids
  • An endpoint whose type is ClusterLoadAssignment does not have name field.
    • Thus, the endpoint name is converted to a cluster name and it's used for retreiving.
    • e.g. groups/foo/endpoints/echo-cluster -> groups/foo/clusters/echo-cluster
  • Each update HTTP API request has {resource}.name for identifying the resource.
  • Thus, the resource name in the resource is ignored. However, update gRPC API uses the resource name.
  • The endpoint name is also converted to a cluster name.

Result:

• You can now create, update and delete xDS resource through gRPC and HTTP APIs.