Validation & shell: pack syntactic checks inside protocol functions

Context

Wrap the plugin's syntactic_check directly into protocol functions, so that we don't need to add the check separately to each call site and don't risk to forget doing so in the future.

Manually testing the MR

CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [N/A] Document any change to the user interface, including configuration parameters (see node configuration)
• [nothing new] Provide automatic testing (see the testing guide).
• [N/A] 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