Proto environment: expose validation functions

Context

This MR refactors (and clean up some of) the environment's interface in order to plug the pipelining features to be usable by the shell. This will change the signatures of block construction(s) and application(s) by exposing a new validation mode which goal is to perform light-weight checks in order to decide whether a block is valid or not.

Manually testing the MR

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)
• [rely on existing tests] 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