kernel/sequencer: add messages definition

Context

The sequencer and the user will send messages to the sequencer kernel. As described in this document, we can list three messages:

• Message: normal message that should be included in the delayed inbox
• Sequence: sequence of messages sent by the sequencer
• SetSequencer: message to rotate/set the new sequencer

The messages are not yet processed

The MR !8875 (merged) is required to be merged before.

You can start reviewing it at the commit "kernel/sequencer: add public key definition"

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR