[SPARK-53467] Improve merge_spark_pr.py to accept PR numbers as a CLI argument

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to improve merge_spark_pr.py to accept PR numbers as a CLI argument.

Why are the changes needed?

To provide an alternative shortcut.

BEFORE

$ dev/merge_spark_pr.py
git rev-parse --abbrev-ref HEAD
Which pull request would you like to merge? (e.g. 34): 235
=== JIRA SPARK-53467 ===

AFTER (Additional usage)

$  dev/merge_spark_pr.py 235
git rev-parse --abbrev-ref HEAD
Start to merge pull request #235
=== JIRA SPARK-53467 ===

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Manual test because this is irrelevant to the CI results.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Since this is a script for committers and the same patch exists in the other Spark repository, I'm going to merge this.

[viirya]

Looks good.

[dongjoon-hyun]

Thank you!

[peter-toth]

Late LGTM.