feat: Support of branches in add_files

[vinjai]

Closes #2428

Rationale for this change

Support to add files to iceberg branches. Currently, you can only add files to the main branch

Are these changes tested?

Yes

Are there any user-facing changes?

New optional paramater for branch in the add_files method of the Table API

[vinjai]

@kevinjqliu @Fokko Request your review

[ForeverAngry]

This pr is 🔥 ! I pulled it down and tested it on a few large tables in my own dev env - and it worked well. Really great work, I'm excited to use this in prod!

[ForeverAngry]

It might be nice to add a negative test that attempts to add files to a non-existent branch, just to make sure that exceptions are handled gracefully and that meaningful errors are surfaced.

[vinjai]

It should be same as the test for appending to non-existing branch:

iceberg-python/tests/integration/test_writes/test_writes.py

Line 2096 in caf02ee

def test_append_to_non_existing_branch(session_catalog: Catalog, arrow_table_with_null: pa.Table) -> None:

If seen from the POV of a snapshot, the flow for add_files and append operation is same as only a new snapshot with new files is being appended.
Since, we are not adding any different code, it just introduces another test which goes through the same flow thus increasing test time.

My suggestion would be to not bloat the test time with similar tests
Thoughts?

[ForeverAngry]

That makes sense to me!

[ForeverAngry]

Great work!

[Fokko]

Nice one @vinjai 🙌 I've left one comment, but apart from that it is good to go 👍

[kevinjqliu]

LGTM

slight nit on the test comment consistency