Bug: Pubsub source requires, but does not use username

[samirketema]

Bug description

The pubsub source has a requirement that username is configured, when it doesn't use it.

A way to resolve this is to have a mode configuration, which can be set to pubsub or streaming.

Steps to reproduce

1. Run a pipeline with the pubsub source without the username configured
2. Note that conduit will throw an error, denoting that the username is required, even though it's unused in the pubsub source.

Version

pubsub branch