feat: Annotate all Rels with output_names

[tokoko]

substrait doesn't keep track of intermediate field names, which makes it hard to properly round-trip queries. Now there's a way to optionally include names in output_names field as a hint attached to every Rel. compiler should populate these fields in order for eventual decompiler to take advantage of them.

[tokoko]

@gforsyth this will probably result in considerably larger proto objects for some queries, so I'm thinking of making the feature optional.

[gforsyth]

Hey @tokoko! I have a few open-ended questions that I think might be useful to flesh out a bit.

Would an eventual decompiler (oh no) require these intermediate field names or merely take advantage of them when they were available?

If they're required, then I think making them optional is going to be a future foot-gun.

I guess this next one isn't a question: I wouldn't worry about making the feature optional until a) we've confirmed how much larger it makes proto objects (probably something like O(num_rels)?) and b) that we have some notion that there is actually a performance penalty worth caring about, like deserialization cost vs query execution time.

[tokoko]

Would an eventual decompiler (oh no) require these intermediate field names or merely take advantage of them when they were available?

I think not. Decompiler should ideally handle all valid substrait plans, not just the ones that we produce, so it should be strictly optional with a fallback to "randomly" generated names.

I agree on the second point. We can make it work first and evaluate performance trade-off later.