Disclaimer: I am still wrapping my head around the initialization code for Veneer clients, so please shout if this doesn't make sense.

This PR is mainly intended to allow users to specify the size of the GRPC channel pool, but it exposes a way to set all options consumed in the venner-generated FirestoreSettings. AFAIK, there was no way to overwrite the TransportChannelProvider or the CredentialsProvider.

Code snippet that sets the max pool size:

Builder options =  FirestoreOptions.newBuilder().build().toBuilder();
options.setChannelProvider(
   FirestoreOptions.getDefaultTransportChannelProviderBuilder().setPoolSize(2).build());
firestore = options.build().getService();

Without this setting, Firestore is limited to 100 concurrent listeners.