Shell: cleaner handling of validator specific RPCs

This simple MR, in the context of !9490 and %Non-blocking RPC feature as experimental, aims to refactor a bit the Shell.Block_directory so that it simplifies the way RPC directories are build, depending on whether or not they depend on the validator component.

On top of !12974 (merged)

Manually testing the MR

Green CI

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