Parameterize Merge Strategy in `gh\pr-merge` action

[iumangg]

Description

The current gh/pr-merge action has a hardcoded --merge flag. This limits the utility of the action for teams that enforce squash or rebase commit histories. We need to update the script to accept the merge method as a parameter.

Tasks

• Add the following input parameter:
  • Merge method
• Implement logic to:
  • Input validation to accept squash and rebase as merge method, for any other input log the input and default to merge
  • Map the input to the corresponding GitHub CLI flag.
  • Default to --merge for backward compatibility.

Expected Outcome

• Parameter Support: The script must accept a new input variable for merge method
• Dynamic Flag Mapping:
  • If squash is provided, the command must use the --squash flag.
  • If rebase is provided, the command must use the --rebase flag.
  • If merge is provided or anything else, the command must use the --merge flag.
• Default Behavior: If the parameter is empty or not provided, the script must default to --merge to avoid breaking existing workflows.
• Input Normalization: The script should handle case-insensitivity (e.g., SQUASH or squash should both work).
• Successful Execution: The PR must be successfully merged using the selected strategy without manual intervention.
• Test: Update the test workflow to test the merge method changes