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)

The issue names no files, tests, or entry points. Start by reading the linked discussion in issue 2428 to determine the intended API and scope, then locate the existing PyIceberg branch operations and tests; done means agreeing on and supporting the five listed strategies.

Written by the indexing model from the issue text.