Proto: Generalize the [destination] argument of [Transaction]

Context

This is a subpart of !4017 (merged), extracted from it to reduce its scope.

The goal is to prepare the use of the Transaction operation to interact with layer-2 solutions the same way we interact with the layer-1 smart contract.

This MR should not change anything feature-wise, but it does include a breaking change: the Transaction_result encoding is not backward compatible (the binary one, JSON should be fine).

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR