Spec: Allow the use of source-id in V3

[Fokko]

Changes around the multi-argument transforms, mainly two things:

• Up for debate. The spec does not point out an actual implementations of transforms that accept multiple arguments. From the existing transforms, the only contender is the bucket transform. Should we include this in the V3 spec? It will only allow to prune metadata if you do an equality expression on all the fields that are part of the transform.
• Along the way, we've removed something that we did not intend. First we allowed to write source-id and source-ids based on the number of arguments. This has been changed to only allow source-ids for V3 in a PR that introduces backward compatibility. I think this makes the JSON parsers/producers more complex than needed (specifically PyIceberg). Also, in Java we would need to plumb down the table version to the PartitionSpecParser.java. I think it would be great to simplify this.

[szehon-ho]

Hi @Fokko i havent take a look yet at spec change, but for multi bucket we had some discussions last year. For reference the pr is here : #8259 with more discussion from: #8579

[szehon-ho]

I wonder should we remove multi-bucket into separate pr, to allow the source-id part to get in?

[Fokko]

@szehon-ho I think that's a good idea. Let me rework the PR 👍

[rdblue]

I think there's a typo, but overall I think this is the right direction. Thanks, @Fokko!

[RussellSpitzer]

Looks good to me, thanks @Fokko !