Replace Digest with a plain array in OnionAnnounceResponsePayload about tox

[Atul9]

@kurnevsky I am new to Rust and this project.
I tried to make the change([u8; 32]) locally and ran tests.

1. I think the size of the array should be 91 since in onion_announce.rs ping_id function makes use of ONION_PING_DATA_SIZE which is 91. I came across this while fixing the failing tests hence I think it should be 91. Please let me know if I should proceed with updating the array size to 91
2. What changes are needed to be done for and convert to PublicKey when needed since ping_id_or_pk is an array after this change.
3. Is data_pk is needed on struct OnionNode after this change? since ping_id_or_pk is not a sha256::Digest or PublicKey after this change.
4. Is ping_id_or_pk(onion_announce_response) is combination of ping_id and data_pk for OnionNode?

[kurnevsky]

1. No, it should be 32 bytes - the length of curve25519 key and sha256 hash. ONION_PING_DATA_SIZE is the size of a data from which we calculate sha256 hash.
2. We treat ping_id_or_pk as a key or as a hash depending on AnnounceStatus from a packet. So we just need to construct key and hash in the right places.
3. Yes, ping_id_or_pk is still either a hash or a key. We just want to store it as a plain array unless we don't know what exactly it is.
4. It's either a hash or a key.