xds: introduce generic xds clients xDS and LRS Client API signatures #14058

Job	Run time
vet-proto	24s
vet-proto	24s
tests (vet, 1.22)	2m 30s
tests (vet, 1.22)	2m 30s
tests (extras, 1.23)	2m 46s
tests (extras, 1.23)	2m 46s
tests (tests, 1.23)	4m 9s
tests (tests, 1.23)	4m 9s
tests (tests, 1.23, -race)	6m 10s
tests (tests, 1.23, -race)	6m 10s
tests (tests, 1.23, 386)	5m 4s
tests (tests, 1.23, 386)	5m 4s
tests (tests, 1.23, arm64)	6m 54s
tests (tests, 1.23, arm64)	6m 54s
tests (tests, 1.22)	4m 16s
tests (tests, 1.22)	4m 16s
tests (tests, 1.23, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	4m 44s
tests (tests, 1.23, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	6m 6s
	1h 15m 16s

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