Proto/SRU: allow Michelson type `contract` in rollup messages

Context

This MR makes Michelson values of type contract _ allowed in messages exchanged between smart contracts and smart rollups. In particular, it opens the possibility of using typed callbacks in outbox messages.

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