Customized resource type extension support

Currently there is limited count of resource types in [xDSv2](https://www.envoyproxy.io/docs/envoy/latest/api-v2/api)/[xDSv3](https://www.envoyproxy.io/docs/envoy/latest/api-v2/api), and should be enough for common scenarios.

But if we have some customized resource types, e.g. private module in envoy which relies on specific configs, then resource type extension support is needed.

Like that the pattern UDPA suggests: separate transport(UDPA-TP) and data-models(UDPA-DM), and it should be easy to extend data models.