Adds automation for generating the map of `gRPC Method Name → Rate Limit Type`
used by the middleware introduced in #15550, and will ensure we don't forget
to add new endpoints.

Engineers must annotate their RPCs in the proto file like so:

```
rpc Foo(FooRequest) returns (FooResponse) {
  option (consul.internal.ratelimit.spec) = {
    operation_type: READ,
  };
}
```

When they run `make proto` a protoc plugin `protoc-gen-consul-rate-limit` will
be installed that writes rate-limit specs as a JSON array to a file called
`.ratelimit.tmp` (one per protobuf package/directory).

After running Buf, `make proto` will execute a post-process script that will
ingest all of the `.ratelimit.tmp` files and generate a Go file containing the
mappings in the `agent/grpc-middleware` package. In the enterprise repository,
it will write an additional file with the enterprise-only endpoints.

If an engineer forgets to add the annotation to a new RPC, the plugin will
return an error like so:

```
RPC Foo is missing rate-limit specification, fix it with:

	import "proto-public/annotations/ratelimit/ratelimit.proto";

	service Bar {
	  rpc Foo(...) returns (...) {
	    option (hashicorp.consul.internal.ratelimit.spec) = {
	      operation_type: OPERATION_READ | OPERATION_WRITE | OPERATION_EXEMPT,
	    };
	  }
	}
```

In the future, this annotation can be extended to support rate-limit
category (e.g. KV vs Catalog) and to determine the retry policy.