xds: introduce simple grpc transport for generic xds clients #14135

Job	Run time
vet-proto	19s
tests (vet, 1.22)	1m 43s
tests (extras, 1.23)	2m 55s
tests (tests, 1.23)	4m 18s
tests (tests, 1.23, -race)	5m 59s
tests (tests, 1.23, 386)	4m 44s
tests (tests, 1.23, arm64)	5m 40s
tests (tests, 1.22)	3m 41s
tests (tests, 1.23, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	5m 50s
	35m 9s

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