Kernel SDK: add external message framing protocol

Context

Originally created in tezos/kernel as part of demo work by @xf6. Moving over here as it's useful for different kernels to follow the same framing protocol, and re-worked slightly.

Currently this is purely optional, and one would have to use it manually on the byte sequence from an external message. In future, additional functions could be added for a higher-level 'parse-once' workflow.

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