Proto, Tx_rollup: layer2 implementation of layer2-to-layer1 withdrawal

Context

Part of #2259 (closed)

Work in progress of the layer2-part of the rollup withdrawal implementation.

Design documents:

• See https://hackmd.io/AlYZVeCrSRK6A_6gFlMX3A?both for the design of withdrawals
• See https://hackmd.io/GaiinMOrRpGunQkQQUXitw for an overview of tx_rollups

Related MRs:

• See !4523 (merged) for the layer1-part of withdrawals

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