Provide Zebra compatibility: re-enable and fix zcash_client_backend, …

…update prost dependency version (#75)

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).

.github/actions/prepare/action.yml

@@ -31,6 +31,7 @@ runs:
         transparent-inputs
         sapling
         unstable
+        unstable-spanning-tree
         ${{ inputs.extra-features }}
         ${{ steps.test.outputs.feature }}
         '" >> $GITHUB_OUTPUT

.github/workflows/ci.yml

@@ -193,9 +193,9 @@ jobs:
       - name: Add zcash_proofs as a dependency of the synthetic crate
         working-directory: ./ci-build
         run: cargo add --no-default-features --path ../crates/zcash_proofs
-#      - name: Add zcash_client_backend as a dependency of the synthetic crate
-#        working-directory: ./ci-build
-#        run: cargo add --features lightwalletd-tonic --path ../crates/zcash_client_backend
+      - name: Add zcash_client_backend as a dependency of the synthetic crate
+        working-directory: ./ci-build
+        run: cargo add --features lightwalletd-tonic --path ../crates/zcash_client_backend
       - name: Copy pinned dependencies into synthetic crate
         run: cp crates/Cargo.lock ci-build/
       - name: Add target