[Fix] Use array for handshake commit hashes

[kaimast]

This PR aims to address the test failures seen in [this run](https://app.circleci.com/pipelines/github/ProvableHQ/snarkOS/2998/workflows/68fb3599-4c04-49b3-810f-2f35b013ff61/jobs/58981.

I believe this error occurs because in the current implementation any string that starts with a ? is considered invalid/unknown, whereas with this change there is exactly one sequence that corresponds to unknown.
So it might encode "?\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", but decoding it will return unknown.

Changes

The PR changes the SHA to always be encoded as a 40-byte array. There was code that converted it from a string, without checking the length, which was error-prone.
Additionally, it relied on a hardcoded "unknown" string, which is now replaced by a constant UNKNOWN_SHA.

An alternative fix would have been to change the protest to not generate a string containing ?, but, in the long-term, it is safer to also ensure we always send exactly 40 bytes.

Reproducibility

I was able to reproduce this on staging by running PROPTEST_SEED=e472a2f4ccb7abe5a6f77dc67ab294386b82e205ea3a889c7707fa15328545c3 cargo test --package snarkos-node-bft-events helpers::codec::tests::event_roundtrip which generates the error below:

thread 'helpers::codec::tests::event_roundtrip' panicked at node/bft/events/src/helpers/codec.rs:108:5:
Test failed: assertion `left == right` failed
  left: [7, 0, 0, 0, 0, 0, 0, 0, 125, 34, 130, 115, 120, 14, 165, 160, 121, 106, 108, 183, 190, 119, 186, 201, 106, 81, 181, 42, 179, 55, 3, 133, 233, 50, 204, 189, 142, 70, 0, 5, 0, 0, 0, 0, 0, 0, 0, 0, 117, 110, 107, 110, 111, 119, 110]
 right: [7, 0, 0, 0, 0, 0, 0, 0, 125, 34, 130, 115, 120, 14, 165, 160, 121, 106, 108, 183, 190, 119, 186, 201, 106, 81, 181, 42, 179, 55, 3, 133, 233, 50, 204, 189, 142, 70, 0, 5, 0, 0, 0, 0, 0, 0, 0, 0, 63, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0].
minimal failing input: input = _EventRoundtripArgs {
    event: ChallengeRequest(
        ChallengeRequest {
            version: 0,
            listener_port: 0,
            address: aleo1053gyumcp6j6q7t2djmmuaa6e949rdf2kvms8p0fxtxtmrjxqqzslunqv7,
            nonce: 0,
            snarkos_sha: "?\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0",
        },
    ),
}

[vicsn]

LGTM! glad you found the issue. After merging this PR, could you duplicate my latest sha fix PR into canary onto staging?