Need ability to set CredentialsProvider programatically

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**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**