New branch discovery strategy: only explicitly listed

[hcorg]

Use case: I want to build only "main" branch and MRs. Existing "Only Branches that are filed as MRs" strategy has built-in bug - it does build branch when there's only one MR. It might be required by it being "legacy" (I've filled issue #761). This MR adds new strategy that filters out all branches except for the ones matching the regex pattern.

As part of the MR I've also extracted duplicated pattern checking code to the base class.

I think that this strategy could replace "Only Branches that are filed as MRs", because it looks like the strategy that one was supposed to be (they behave the same when there are more than one MR, or to be precise - MRs with various source branches). But maybe the "legacy behaviour" is important. Let me know - I can change the PR to replace old strategy.

Testing done

My local instance of the Jenkins runs with the option enabled and it works as expected:

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed