Proto validation of gRPC runtime credentials

In some places, e.g. https://github.com/envoyproxy/envoy/blob/7267542177ef49b33d6dd7b6ae6cdaa4ce2fb9ce/source/extensions/grpc_credentials/file_based_metadata/config.cc#L31, https://github.com/envoyproxy/envoy/blob/7267542177ef49b33d6dd7b6ae6cdaa4ce2fb9ce/source/extensions/grpc_credentials/aws_iam/config.cc#L39, we're explicitly plumbing in a null proto validation visitor, rather than using something derived from server settings. We also, more generally, lack handling of proto validation failure.

In some sense, this is totally fine today, since these are not control plane API and don't use PGV. In principle, it might be useful to have unknown field validation here and PGV support, so opening this issue to track future work here.