Add branch merge strategies

### Feature Request / Improvement

Add merge strategies commonly used in version control systems to pyiceberg. Motivated by some of the discussion found here https://github.com/apache/iceberg-python/issues/2428:

1. **MERGE**: creating a merge commit that preserves history of both branches
2. **SQUASH**: condenses all commits from source branch into a single clean commit on target branch
3. **REBASE**: creates linear history by replaying commits from source branch on top of target branch
4. **CHERRY_PICK**: selects and applies specific individual commits from one branch to another
5. **FAST_FORWARD**: moves target branch pointer forward when no divergent commits exist (no merge commit needed)