Support "squash and merge" in Merge Train

Problem to solve

Currently, Merge Train doesn't allow users to choose squash option. It could be a deal-breaker for some users. We should consider supporting squash and merge.

Intended users

Developers

Permissions and Security

To squash and merge pull requests, you must have write permissions in the repository, and the repository must allow squash merging.

These actions should be logged to audit log

Feature Flag

This feature is implemented behind merge_train_new_stale_check feature flag and enabled by default. Once we've confirmed the feature is deemed stable, we remove the feature flag.

Links and references

MRs:

• Preserve merge train rows after merge !19864 (merged)
• Proactively marking stale status in merge train !19434 (merged)
• Documentation update (https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/#requirements-and-limitations)