Tx_rollup: Introduce the storage for inboxes

Context

This is a subpart of !4017 (merged), aimed at reducing the scope of this MR.

While this MR does not bring any particular test (except for the fees logic), its effect if merged in master would be null, because the storage is not used anywhere.

Note that !4017 (merged) on the contrary is using these tables and provides all the test you might want (at least we hope so).

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR