feat(encoding): Add Parquet encoding support

[zhangfengcdt]

This commit introduces Parquet encoding support to the Prolly Tree library.

Changes include:

• Added the parquet crate as a dependency.
• Updated the EncodingType enum to include Parquet.
• Implemented the encode_parquet function.
• Added a test case for Parquet encoding and made tests order-agnostic.
• Bumped the version to `0.2.0-alpha.1.

[Copilot]

Pull Request Overview

This PR introduces Parquet encoding support to the Prolly Tree library by adding a new encoding variant, implementing serialization logic, and updating dependencies and tests accordingly.

• Added Parquet variant to EncodingType and wired it through encode_pairs.
• Implemented encode_parquet with Arrow-to-Parquet serialization.
• Added a test for Parquet encoding and updated Cargo.toml for new dependencies.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/encoding.rs	Added Parquet import, enum variant, encode_parquet method, and corresponding test logic.
Cargo.toml	Bumped version to 0.2.0-alpha.1 and added parquet & bytes dependencies.

Comments suppressed due to low confidence (2)

src/encoding.rs:73

• [nitpick] Consider changing the signature to return Result<Vec, ParquetError> (or a more general error type) so that encoding failures can be propagated instead of panicking.

    fn encode_parquet(&self) -> Vec<u8> {

src/encoding.rs:386

• [nitpick] It may be helpful to add tests for edge cases (e.g., empty inputs or single-row batches) to ensure encode_parquet behaves correctly in all scenarios.

    fn test_encode_parquet() {