Add DEFAULT sentinel for gapic_v1.method

[theacodes]

This is needed because the wrapped gapic methods can't discern between "unspecified" and "None" because the gapic clients will always specify retry and timeout, e.g.:

def get_operation(name, retry=None, timeout=None):
     request = operations_pb2.GetOperationRequest(name=name)
     return self._get_operation(request, retry=retry, timeout=timeout)

gapic methods will use this sentinel to indicate unspecified:

def get_operation(name, retry=gapic_v1.method.DEFAULT, timeout=gapic_v1.method.DEFAULT):
     request = operations_pb2.GetOperationRequest(name=name)
     return self._get_operation(request, retry=retry, timeout=timeout)

[dhermes]

LGTM