Tezlink/Node: implement balance RPC

What

Implement the /balance rpc.

previous: !17615 (merged)

Why

We only provided a mock version when adding the rpc. Now we need the backend.

How

We take a similar approach to etherlink by inserting a module into Service_backend_sig.S, by adding a new module Tezlink_storage.

• This avoid having two way to do the same thing.
• while still cleanly separating etherlink specific parts of the backend from tezlink specific parts.

Contrary to what was tried before, the protocol imports are not private and not all in the same place: we have added a Tezlink_imports module to the encoding lib. Not sure if that was a valid concern in the first place, and what would be the best approach if it was. In any case, we'll need to be vigilant in reviews to avoid refering to the protocol directly.

Manually testing the MR

We already have a test for balance. Proper testing is done only using the bootstrap accounts for now.

We will rebase on !17536 (merged) when it's in master.

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