[Variant] Initial API for reading Variant data and metadata

[mkarbo]

Which issue does this PR close?

• Closes [Variant] Rust API to Read Variant Values #7423

Rationale for this change

We need to agree on an API for reading Variant metadata. Based on the work and discussions in #7452, in this PR we propose an API plus an implementation (WIP while draft) for reading variant metadata in the parquet-variant crate.
A lot of the work is based on the work in #7452 by @PinkCrow007 and feedback from @alamb, @scovich, and @Weijun-H.

What changes are included in this PR?

• Adds Variant enum (and associated structs)
• Adds an API for parsing and reading metadata
• Adds an API for parsing and reading Variant values of various types

We attempt to be result- and validation driven while ensuring zero-allocations, and we do so avoiding serde_json.

We tried to keep the Variant API similar to the Json::Value api.

Are there any user-facing changes?

The new API's added in parquet-variant will be user facing.