Context

What

We don't want to force users to read gRPC docs and construct a Grpc::ChannelCredentials in the common situation that they want to use a keyfile.

How

Allow the initialize method of the client object to accept either a string representing the path to a keyfile, or a Grpc::ChannelCredentials as the value to chan_creds. If a string is received, GAX should construct the Grpc::ChannelCredentials automatically.

cc: @landrito @eoogbe