Add Refs metadata table

[geruh]

This PR adds the Refs metadata table the existing inspect logic for Iceberg tables as listed in #511. The refs metadata table in Iceberg stores the table's known snapshot references including branches, and tags.

Usage

> table.inspect.refs() 

pyarrow.Table name: string not null type: string not null snapshot_id: int64 not null max_reference_age_in_ms: int64 min_snapshots_to_keep: int32 max_snapshot_age_in_ms: int64 ---- name: [["testBranch","main","testTag"]] type: [["BRANCH","BRANCH","TAG"]] snapshot_id: [[4549716733136159472,4549716733136159472,4549716733136159472]] max_reference_age_in_ms: [[604800000,null,15552000000]] min_snapshots_to_keep: [[2,null,null]] max_snapshot_age_in_ms: [[null,null,null]] 

[Fokko]

Looking good @geruh. I've left one small comment 👍

[Fokko]

What do you think of changing this field into a categorical? https://pandas.pydata.org/docs/user_guide/categorical.html#series-creation

[geruh]

Hey Fokko, I looked into this for both pandas and pyarrow and it offers a few benefits like reduced memory usage and improved performance on sorting and filtering. Since we're only dealing with branch or tag, this is ideal. However, this does stray from the Java implementation which uses a string type. But I think we should be fine with this difference, because in both pyarrow and pandas, this type seems to be well supported and can be easily converted to string if needed. Also, this would be beneficial for huge tables with many references.

In pyarrow, this would be implemented as a dictionary type mapping integers to strings. Also, this could be added to the other metadata tables such as the operation field in the snapshots table.

[Fokko]

Thanks, and yes, I agree that it is okay to diverge from the Java implementation here since it is just an implementation detail.

[amogh-jahagirdar]

Looks great, just a nit in the test from my side!

[Fokko]

@geruh Thanks for creating this PR! Can you resolve the conflicts so we can get this in? Thanks!

[Fokko]

Thanks @geruh for working on this 🙌