DAL/node/Store: add interface file store.mli

What

Closes #4097 (closed).

Why

The store of the DAL node maintains some invariants, in particular it provides a bidirectional mapping between commitments and slot identifiers implemented as a pair of mappings which must stay in sync. Adding an interface in which the allows to make the store abstract to better guarantee that the invariants cannot be broken outside of the module.

How

Manually testing the MR

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