Convert JSON to VariantArray without copying (8 - 32% faster)

[alamb]

Which issue does this PR close?

• part of [EPIC] [Parquet] Implement Variant type support in Parquet #6736
• Closes [Variant] Convert JSON to Variant with fewer copies #7964
• Follow on to Add VariantArray and VariantArrayBuilder for constructing Arrow Arrays of Variants #7905

Rationale for this change

In a quest to have the fastest and most efficient Variant implementation I would like to avoid copies if at all possible
Right now, to make a VariantArray first requires completing an individual buffer and appending it
to the array.

Let's make that faster by having the VariantBuilder append directly into the buffer

What changes are included in this PR?

1. Add VariantBuilder::new_from_existing
2. Add a VariantArrayBuilder::variant_builder that reuses the buffers

Are these changes tested?

1. New unit tests
2. Yes by existing tests

Are there any user-facing changes?

Hopefully faster performance