Make it easier to use non defaults CredentialsProvider

[gpoulin]

Is your feature request related to a problem? Please describe.
To be able to set a CredentialsProvider on a SubscriberSettings, you need to set the credentials on a Supplier<SubscriberServiceClient>, a Supplier<CursorServiceClient> and a PartitionAssignmentServiceClient

Describe the solution you'd like
Be able to set CredentialsProvider on the SubscriberSettings directly.

[dpcollins-google]

Hi Guillaume,

Trying to figure out how to prioritize this: Are you unable to use default credentials either through GOOGLE_APPLICATION_CREDENTIALS or machine defaults on Compute Engine? If so, why? If we implemented this, we'd probably want to do it across other languages as well.

-Dan

[gpoulin]

Do facilitate our migration to GCP, we will need to be able to publish messages to Pub/Sub from our data center. In our infrastructure, the local filesystem can't be considered secure . For other GCP client, we simply base64 the credential file and put he result as an environment variable. Creating a CredentialsProvider from that is pretty trivial.

In addition, we have used impersonated credentials from time to time.

[dpcollins-google]

Thank you for the scope as to why this is necessary. I'll ensure this is implemented in all clients.