fix: use AuthMetadataPlugin with context manager

[busunkim96]

If this is merged, google-api-core will have to be modified to make use of the context manager.

@lidiz @gnossen Does this look reasonable?

[gnossen]

@busunkim96 There are also currently no hooks in gRPC to call your __enter__ and __exit__ methods. We've have to make changes to the gRPC Python-level creds API to make this work. What memory leak remains after #461? If the creds objects are not being garbage collected ever, then there's likely a reference cycle issue that needs to be addressed.