Update the DMQ message structure

[jpraynaud]

Why

The message structure in the DMQ node specifications has evolved and it needs to be updated in the documentations and implementations:

• remove the blockNumber field
• remove the ttl field
• add an expiresAt field (word32)
• add a coldVerificationKey field (word32)

What

Document and update the new DMQ message structure.

How

• Update the Haskell node (already done)
• Update the CIP-0137:
  • Prepare a PR
    • remove the blockNumber field
    • remove the ttl field
    • add an expiresAt field (word32)
    • add a coldVerificationKey field (word32)
    • explain the units used for the expiresAt
    • notification n2c fix typos in StBusy in agency table
    • Specify the fields to be signed by the KES keys
    • Split message payload and signature parts
    • Enhance documentation for authentication (bytes encodings, ...)
  • Ask for review for the PR
  • Merge the PR
• Update the Pallas library with the DMQ message
  • Prepare a PR
  • Ask for review for the PR
  • Merge the PR
• Update the Mithril nodes:
  • Adapt the structure of the DMQ message
  • Update the computation of the KES signature of the message
  • Enhance the implementation of the DMQ consumer server queue to enforce removal of messages based on the expiration date

Later

• Prepare test vector for message authentication