Provide Zebra compatibility: re-enable and fix zcash_client_backend, update prost dependency version

[dmidem]

Updates the repository to ensure compatibility with the zebra project.

Changes:

1. Uncommented the zcash_client_backend crate in the root Cargo.toml. Initially it was commented out under the assumption that it wasn't utilized by Zebra. However, it has been identified that Zebra relies on this crate.

2. Synchronized zcash_client_backend crate with zcash_note_encryption updates.

3. Updated the prost dependency version in the root Cargo.toml (the pinned version 0.12.3 caused compatibility issues with Zebra’s dependencies, preventing compilation).