Implement `AttestationManager`

All the offers received by the `BoardManager`, independently of the communications channels (BOLT4 onions, BOLT8 noise channel or NIP 01 websocket) and sources (clients or peers).

The `AnchorManager` (in `anchormanager.rs`) should accept serialized offer payload from the `BoardManager`, pack them in an anchor and at periodic intervals (e.g every X blocks or clock ticks), counter-sign them with the board public key and submit for validation to the selected Mainstay server.

The anchor format should be some metadata (version, time, previous anchor SHA256 hash if proof-of-immutable-sequence is opted-in) and a byte payload.

The Mainstay protocol is documented here: https://commerceblock.readthedocs.io/en/latest/mainstay-background/index.html and the original paper is available here: https://cloudflare-ipfs.com/ipns/ipfs.commerceblock.com/commerceblock-whitepaper-mainstay.pdf