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)

此 issue 未指定檔案、測試或進入點。先閱讀 issue 2428 中連結的討論，以確定預期的 API 和範圍，然後找出 PyIceberg 現有的分支操作和測試；完成的標準是就列出的五種策略達成共識並予以支援。

由索引模型根據 Issue 內容生成。