[Feature Request]: PubsubMessageMatcher Credentials Argument

[mchinbox]

What would you like to happen?

I would like the PubsubMessageMatcher class in Python to support handling a credentials object. In my situation, I do not have write access nor do I want to create a temporary JSON to set Google Application Default Credentials. I am able to work around this with Google Client Service Account Credential objects when handling these clients myself, but the client instantiated in PubsubMessageMatcher assumes you're authenticated through the environment variable (or so I think?)

Issue Priority

Priority: 3 (nice-to-have improvement)

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