Split out variant code into several new sub-modules

[scovich]

Which issue does this PR close?

Housekeeping, part of

• [EPIC] [Parquet] Implement Variant type support in Parquet #6736

Rationale for this change

The variant module was starting to become unwieldy.

What changes are included in this PR?

Split out metadata, object, and list to sub-modules; move OffsetSize to the decoder module where it arguably belongs.

Result: variant.rs is "only" ~900 LoC instead of ~2kLoc.

Are there any user-facing changes?

No. Public re-exports should hide the change from users.

[scovich]

NOTE: The code movement exposed some parts of the code that have no direct unit tests:

• The utility methods in utils.rs (but heavily used by other modules)
• The Variant struct itself in variant.rs (perhaps covered by the interop tests)

If we think the above represents an actual test gap, we may want to file some (good first) issues?

[alamb]

YES ! thank you @scovich

[alamb]

If we think the above represents an actual test gap, we may want to file some (good first) issues?

Sounds good to me

Note that doc examples I think function as unit tests as well, so we can include them too

[alamb]

Thank yoU @scovich

I'll fix the docs CI test and then merge this one in as it has a high potential to attract conflicts in other related PRs

[scovich]

I'll fix the docs CI test and then merge this one in as it has a high potential to attract conflicts in other related PRs

Whoop, I didn't see this. Just pushed a fix for the doc tests.

[scovich]

... or rather, just attempted to push; your fix won the race

[alamb]

Looks like I missed another one 🤦