move authorities under xds client #951

# to view logs

Job	Run time
tests (vet, 1.22)	2m 5s
vet-proto	21s
tests (extras, 1.23)	2m 47s
tests (tests, 1.23)	3m 51s
tests (tests, 1.23, -race)	5m 42s
tests (tests, 1.23, 386)	4m 47s
tests (tests, 1.23, arm64)	6m 46s
tests (tests, 1.22)	3m 32s
tests (tests, 1.23, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	5m 40s
	35m 31s

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