Draft: Proto,sc_rollup: Introduce 'partial_reveal_enable' flag

Context

We introduce a new feature flag for a new kind of reveal instruction for the PVM: instead of revealing the preimage of a hash, one can reveal part of the preimage. We thus can get rid of the bound of 4KiB on the length of a preimage.

We add a new host function to reveal part of a preimage.

Manually testing the MR

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