Need ability to set CredentialsProvider programatically

[Xsixteen]

Thanks for stopping by to let us know something could be better!

Is your feature request related to a problem? Please describe.
I am only able to use the pubsub lite - kafka if I set the VM scopes / IAM service account. The other pubsub lite library (https://github.com/googleapis/java-pubsublite) allows the setting of credentials via setCredentialsProvider()

Describe the solution you'd like
I would like to be able to set my credentials programatically via a method similar to setCredentialsProvider()`

Describe alternatives you've considered