feat: Verify conda package's sha256 or md5 from pixi.lock

[Jeff Zhang (e8035669)]

Motivation

closes #217

Changes

• Explicitly call tokio::File::flush() before dropping the File object, as the documentation notes that "a file will not be closed immediately when it goes out of scope".
• Use rattler_digest::compute_file_digest() to verify the file checksum (SHA-256 or MD5) when a corresponding digest is available.
• Enabled tokio feature for rattler_digest to enable async I/O support.
• Use rattler_digest::HashingWriter to verify the file checksum (SHA-256 or MD5) when a corresponding digest is available.

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If updating documentation:

• Updated documentation in https://github.com/prefix-dev/pixi/blob/main/docs/deployment/pixi_pack.md as well