Improve Oracle connector tests on GitHub Actions

Currently integration tests for the Oracle connector cannot be run on pull requests when the user has no credentials setup to fetch the Oracle server container image from the private repo on quay.io. For this there is a check if the credentials are available. But the tests always passes. This has the effect that only PRs originating from the main debezium/debezium repo can run the test suite and it will fail for PRs from forks.

I suggest we enable the full credentials check and based on the result we either run only unit tests (not integration tests) when no credentials are available or when credentials are available we run the whole test suite including ITs.

Alternatively we could think about enabling parts of the build process to run as `pull_request_target` which might have access to the credentials. But we need to find a way to make this in a secure way. Maybe a separate build step using this can just download the artifact for other build steps?

based on https://github.com/debezium/debezium/pull/4917#discussion_r1379706762

FYI jpechane ccranfor@redhat.com