Proto,tx_rollup: Introduce the `tx_rollup_l2_address' type to Michelson

Context

Offspring MR of !4017 (merged) whose goal is to make future rebase of !4017 (merged) easier. If I see this is taking too much time to be merged, I will probably close it in favor of letting everything in !4017 (merged).

Manually testing the MR

!4017 (merged) is making use of this commit. If the CI of the parent MR is green, it probably means we are fine.

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