Sequencer: Sign sequencer kernel messages

Context

Previous MR: !9098 (merged)

Next MR: !9447 (closed)

Blocked by:

• Sequencer kernel MR: !9082 (merged)

This MR introduces signatures of Sequence messages and adds tezt which checks that sequencer kernel successfully parsed & verified message and signature.

Basically, the first commit is cherry-picking the tip of this MR which is ahead of the blocking one, review should start from the second commit (which is supposed to be only one for review).

Manually testing the MR

Build the last version of the kernel first:

rm -rf sequenced_kernel.wasm && make build-kernels

Build the last version of the sequencer node and run tezt:

make octez-smart-rollup-sequencer-node && dune exec tezt/tests/main.exe -- --file sc_sequencer.ml --verbose

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