Tx_rollup: Introduce commitment repr and storage

Context

This is the the first of a planned Merge Request #3.A (see the related document for more information). Changes from !3921 (closed):

• Added storage for: commitments
• Added a RPC call to retrieve commitments.
• Add a L1 operation to create a commitment

This is the first section of !4162 (closed) -- about half of its first and second commits, and all of its third. It depends on tx_rollup@inbox-linked but once !4332 (merged) is merged, we can retarget it to master.

Manually testing the MR

There are unit 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