Kaitai: Introduce user defined types

Context

Closes #6258 (closed). Follows !9933 (merged).

This MR does:

• Introduce missing regression tests for octez-bin-codec-kaitai: 8c758ade -> 8c758ade
  • Not needed for this MR, but to avoid overhead of extra incredibly small MR...
• Code quality improvements/setting the stage:
  • Next 4 commits.
• We add support for user defined types and use them to make more idiomatic kaitai spec files for string and bytes
  • Last commit: c7de35c1 -> 9155bd80

Follow-up:

!9924 (merged)

Manually testing the MR

CLI should pass. Essentially this is only setting up the stage for !9924 (merged) + code quality + extra tests.

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