Runtime: Support path filter in ListResources

[begelundmuller]

Adds support for optionally passing a path filter to ListResources. If provided, it will only return resources created from a file matching the path.

This is implemented in ListResources since we support creating multiple resources from a single file (though that feature is not currently used).

The new request signature is:

message ListResourcesRequest {
  // Instance to list resources from.
  string instance_id = 1;
  // Filter by resource kind (optional).
  string kind = 2;
  // Filter by resource path (optional).
  string path = 3;
}