Add type to smart rollup outbox message

Context

The current outbox message type of Mumbai allows a user to trick the kernel into not withdrawing their funds in the L2 and withdrawing a ticket of the rollup in the L1. This is possible because the expression sent to the L1 is typed against the receiver l1 contract. An expression, e.g. Pair Kt1... (Pair "I'm not a Ticket!" 450), can be typed as either a ticket string or a pair string (pair string nat).

The user can submit to a kernel to send an outbox message Pair (Pair %S "%s") (Pair 450 "Hello, Ticket!) of type pair string (pair string nat) to a contract with type ticket string and so the kernel won't withdraw the user fund in the L2 but still lose the ticket in the l1.

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