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)

该 issue 未指定文件、测试或入口点。首先阅读 issue 2428 中链接的讨论，以确定预期的 API 和范围，然后定位 PyIceberg 现有的分支操作和测试；完成的标准是就列出的五种策略达成一致并予以支持。

由索引模型根据 Issue 内容生成。