[Variant] Add (empty) parquet-variant crate, update parquet-testing pin

[alamb]

Which issue does this PR close?

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

Rationale for this change

We are adding support for Variant and thus we should add a new crate where the new code will live and that clearly explains the current status.

What changes are included in this PR?

1. Add a new parquet-variant crate
2. Add README
3. Update parquet-testing submodule to include variant test data added in Add example binary variant data and regeneration scripts parquet-testing#76

Are there any user-facing changes?

Not yet

[alamb]

@crepererum or @wjones127 or @mbrobbel I wonder if you have a few minutes to review this PR. I hope it is non controversial.

cc @Weijun-H @tustvold @etseidl @PinkCrow007

[wjones127]

Looks good, though I think the readme description could be confusing to some as is.

[adriangb]

Seems like an unnecessary change

[alamb]

I think it is something my editor does by default.

[alamb]

Thank you @wjones127, @mbrobbel and @adriangb for the reviews

[alamb]

I just found that the parquet-java implementation also has something called parquet-variant:

https://github.com/apache/parquet-java/tree/master/parquet-variant/src

Which was added a few days ago with @gene-db:

• GH-3201: Implement a Variant builder to create Variant values parquet-java#3202

[alamb]

Ok, let's merge this to keep the code flowing. I tried to address @wjones127 's comment, but if it is still confusing I would be more than happy to make a follow on PR to further clarify