Allow xds to be a valid endpoint for grpc

Is your feature request related to a problem? Please describe.
I am trying to load-balance my traces from pod to otel-collectors, for this I want to be able to use the grpc-proxyless setup with grpc-xds, but I am getting an error where the xds name resolver is not considered a valid protocol.

Describe the solution you'd like
xds should be considered a valid endpoint protocol so that we can use grpc-proxyless to load-balance traffic to otel-collector

Describe alternatives you've considered
I have tried shifting to http, but grpc-proxyless setup is more resilient as it provides more robust functionalities like circuitbreakers and dynamic endpoint updations.

Additional context
Currently since we are using the grpc protocol, we are connecting to only one of the pods of otel-collector, we want to be able to make our infra more resileient and load-balanceable for which we would need this setup. I believe there not much should be required from otel side except for just supporting the protocol, rest I believe if the grpc-xds library is present in the classpath, we should be good to go.

Start by locating the endpoint protocol validation used by the grpc-proxyless setup and the xds name resolver path described in the issue. Done means xds is accepted as a valid endpoint protocol and grpc-xds can support load balancing to otel-collectors without the current protocol error.