Scoru, Tests: Add encodings for external tx kernel messages

Context

This issue describes the problem pretty extensively #4951.

This is the first part that actually implements encodings of most TX kernel external operations (except for DAC message), and also amends the existing tezt to actually use all of them. This MR doesn't implement DAC message encoding yet, I guess it will be necessary to supply around 10K-100K txs (now it's my current goal) to the kernel. Unless block throughout will be sufficient to see the difference in performance.

Each commit of this MR adds new types of messages gradually, the last one makes an effort to make things better typed.

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