Label MRs based on Source / Target Branch Name

[marhali]

Add new configuration options to label merge requests based on:

• sourceBranchNames Record<string, string[]>
  • Assign labels based on the source branch name of the merge request.
  • Read from CI_MERGE_REQUEST_SOURCE_BRANCH_NAME
• targetBranchNames Record<string, string[]>
  • Assign labels based on the target branch name of the merge request.
  • Read from CI_MERGE_REQUEST_TARGET_BRANCH_NAME