Add support for VPC direct connect

[davidferguson-telana]

Please add support for VPC direct connect / Direct VPC Egress to the Cloud Functions for Firebase Python SDK. At the moment it seems that only support for Serverless VPC Access connectors exists.

Support was added to the Firebase CLI in firebase/firebase-tools#10033 and Firebase SDK for Cloud Functions in firebase/firebase-functions#1823.

[cabljac]

Going through the backlog of open issues. This is a clear parity gap with the Node SDK, and the prior art you linked (firebase/firebase-functions#1823 and the CLI support in firebase/firebase-tools#10033) makes the shape of the change pretty well defined for the Python SDK. Can't promise a timeline, but it's a well-scoped request and it's on our radar.