Add branch merge strategies

[ForeverAngry]

Closes #2433

Rationale for this change

This PR adds comprehensive branch merge strategies to PyIceberg, bringing Git-like branch merging capabilities to Iceberg table operations. This enhancement enables users to merge branches with different strategies depending on their workflow needs.

Feature Overview:
Apache Iceberg supports branch operations (create, delete, tag), but lacked merge capabilities between branches. This PR implements 5 standard merge strategies commonly used in version control systems (note, there are differences between this and the java implementation):

1. MERGE: Classic three-way 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)

Implementation Details:

• Strategy Pattern: Clean, extensible architecture with abstract base class and concrete implementations
• Automatic Detection: Fast-forward possibility automatically detected and validated
• Robust Utilities: Common ancestor finding, branch validation, and snapshot traversal utilities
• Flexible API: Optional source branch deletion after successful merge
• Error Handling: Comprehensive validation with clear error messages for invalid operations

Use Cases:

• Development Workflows: Feature branch integration with different merge policies
• Data Pipeline Management: Merging experimental data processing branches back to production
• Schema Evolution: Combining schema changes from different development branches

Are these changes tested?

Yes, extremely somewhat comprehensive test coverage with 35 tests across multiple categories:

Are there any user-facing changes?

Yes - New Feature Addition (No Breaking Changes)

New Public API:

from pyiceberg.table.update.snapshot import BranchMergeStrategy

# New enum with 5 merge strategies (im open to suggestions on this, I couldn't decided on the best approach)
BranchMergeStrategy.MERGE
BranchMergeStrategy.SQUASH  
BranchMergeStrategy.REBASE
BranchMergeStrategy.CHERRY_PICK
BranchMergeStrategy.FAST_FORWARD

# New method on ManageSnapshots
table.manage_snapshots().merge_branch(
    source_branch="feature",
    target_branch="main", 
    strategy=BranchMergeStrategy.SQUASH,
    delete_source_branch=False  # Optional: preserve or delete source branch
).commit()

[ForeverAngry]

@jayceslesar i know you have done a good bit of work in the managed snapshots class, can you review this as well?

[ForeverAngry]

@gabeiglio i noticed you had created a pretty great proposal for this work. It would be awesome if you wanted to review the implementation and help get it in good shape!

[gabeiglio]

Thanks for the PR! this is awesome, will review it by this week!

[github-actions]

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[github-actions]

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