Also provide an API that automatically load KmsClient impls annotated
with @AutoService using java.util.ServiceLoader. Tinkey will use this
API to talk to custom, in-house KMS.

Change-Id: Id1cabb6a17df96597afb10f3b4bd1f9331bc0c67
ORIGINAL_AUTHOR=Thai Duong <thaidn@google.com>
GitOrigin-RevId: a6a18a88fef36b17c58b4bc4e18df91d73e94ffa