Go SDK: add dag.If with Then and Else so native Dags can branch

[FrankYang0529]

Implement Go SDK: Add dag.If with Then and Else.

• dag.If(fn, opts...) returns an *airflow.IfRef and adds a task that runs the condition function fn. fn returns (bool, error). It follows the same rules as a function passed to dag.Task. IfRef.Then and IfRef.Else take the *airflow.TaskRef of the task to run on each side.
• When fn returns true, the condition task skips the task from Else. When fn returns false, the condition task skips the task from Then. When fn returns an error, the condition task fails.
• Before fn runs, the condition task writes {"skipped": []} to the skipmixin_key XCom. NotPreviouslySkippedDep reads that XCom when someone clears a skipped task. The Go runtime does not delete the XComs of earlier tries. Without the empty list, a list from an earlier try could skip a task that the current try did not skip. When the condition task skips a task, it first writes the task_id of that task to the skipmixin_key XCom and then sends SkipDownstreamTasks.

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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[jason810496]

Nice, thanks.
No major finding from me, here're the comments caught by the Claude reivew.

[jason810496]

Thanks