…or requests transport (#390)

This commit includes the following changes:
- `transport.grpc.AuthMetadataPlugin` is now non-blocking as gRPC requires
- `transport.requests.Request` now has a default timeout value of 120 seconds so that token refreshing will not be stuck

Resolves: #351

… is not encoded (#393)

* fix: in token endpoint request, do not decode the response data if it is not encoded

The interface of the underlying transport
'google.auth.transport.Request' that makes the token
request does not guarantee the response is encoded. In Python 3, the
non-encoded strings do not have 'decode' attribute. Blindly decoding all
the response could have the token refresh throw here.