Initial Builder API for Creating Variant Values (#7653)

# Which issue does this PR close?

- Closes #7424
- Part of #6736

# Rationale for this change

This PR introduces a basic builder API for creating Variant values,
building on the foundation laid by @mkarbo. The builder provides a
user-friendly nested API while maintaining performance through a
single-buffer design. The design was shaped with huge help from @alamb,
@scovich and @Weijun-H ’s feedback, and draws much inspiration from the
excellent work by @zeroshade

This is an initial version and does not yet support nested values,
metadata key sorting, and so on

# What changes are included in this PR?

- Adds VariantBuilder, ObjectBuilder, ArrayBuilder

# Are there any user-facing changes?

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

---------

Co-authored-by: Andrew Lamb <[email protected]>

Author: PinkCrow007

parquet-variant/Cargo.toml

@@ -28,7 +28,9 @@ authors = { workspace = true }
 keywords = ["arrow", "parquet", "variant"]
 readme = "README.md"
 edition = { workspace = true }
-rust-version = { workspace = true }
+# needs a newer version than workspace due to
+# rror: `Option::<T>::unwrap` is not yet stable as a const fn
+rust-version = "1.83"
 
 [dependencies]
 arrow-schema = "55.1.0"