[Variant] Introduce parquet-variant-integration-testing crate

[alamb]

Which issue does this PR close?

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• Part of [Variant] Integration tests for reading parquet w/ Variants #8084
• Follow on to [VARIANT] Initial integration tests for variant reads #8104

Rationale for this change

We are trying to avoid adding new dependencies into the parquet crate while adding Variant support

However, eventually to use Variants with Parquet, a user will need to use code from both the parquet crate and the parquet-variant and parquet-variant-compute crates. To so I propose adding a new parquet-variant-integration-testing crate`

What changes are included in this PR?

1. Add a new parquet-variant-integration-testing crate
2. Move the test from [VARIANT] Initial integration tests for variant reads #8104 by @carpecodeum to this new crate
3. Add new crate to various build process

Are these changes tested?

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2. Serve as another way to document the expected behavior of the code

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Are there any user-facing changes?

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[alamb]

I thought a bit about this approach, and the more I think about it, the more I am thinking it might be simpler to put this test in the parquet-variant-compute crate 🤔 I will ponder

[alamb]

Actually, the more I think about this, the more I think maybe we want a variant feature as part of the parquet crate. That will be much easier to use than having to use a separate crate. I'll make a PR to see what that would look like