Tezlink/Node: balance service

What

Adds a .../balance rpc, with a dummy answer.

Why

It's a first step toward the ledger goal.

How

Redefine a rpc and register it. We can't import it directly, because the Contract_services.S module defining the services is private (only their implementation are public).

In parallel, we're discussing with L1 what is the best way to procede to make those services accessible.

Manually testing the MR

dune exec etherlink/tezt/tests/main.exe -- tezlink balance

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