Automate manual QA instructions for OAuth integration tests

[npepinpe]

Description

When we initially implemented the OAuth credentials provider (both for Go and Java), me and Miguel added some helpers to run integration tests against real OAuth2 providers under clients/oauth2, with some instructions on how to use them. These would setup what we expected would be similar to a production set up - a reverse proxy in front of Zeebe performing the authentication, and an OAuth2 provider.

This helped us catch two bugs after the initial release, so it was useful, but it hasn't really be used ever since and most likely is outdated at this point. It also doesn't help us prevent further bugs since we don't run this as part of our normal QA.

I would propose replacing the tests with just a simple integration test for each client that tests a plain OAuthCredentialsProvider (in Java and in Go) against a real provider, likely hydra as it's the lighter one (Keycloak takes a good 30s to just start up versus hydra which takes a couple).

[npepinpe]

Leaving it in the backlog for now as there is no major changes planned for the Java/Go client at the moment.