Sign security parameter offset in ProtocolMessage for Blocks and Transactions

[jpraynaud]

Why

We want to sign with Mithril the offset from the tip of the chain used to create the snapshot. This will allow the clients to rely on a verified information when assessing if the blocks/transactions verified are immutable enough to be trusted given their security requirements.

What

Add the offset from the tip of the chain in the Protocol Message signed for the Blocks and Transactions.

How

• Create a new BlockNumberOffset representing the offset from a block number:
  • Create the new type (not alias, follow the implementation done for KesPeriod and KesEvolutions)
  • Implement arithmetic operations between BlockNumber and BlockNumberOffset
  • Implement Deref and From traits
  • Update the block number offsets to the BlockNumberOffset type when they are using BlockNumber
• Update the Signed entity type CardanoBlocksTransactions to include a BlockNumberOffset
• Update the generic type used by the signable builder from U=BlockNumber' to U=(BlockNumber,BlockNumberOffset)
• Add a new variant for the ProtocolMessagePartKey: CardanoBlocksTransactionsBlockNumberOffset
• Sign the BlockNumberOffset in the compute_protocol_message of the signable builder implementation
• Update the client library
• Run a backward compatibility test with previous distribution 2603.1