Proto: validators RPC also returns companion key

This MR adds a companion key to validators RPC:

./octez-client rpc get '/chains/main/blocks/head/helpers/validators?delegate=tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx&level=9'
[ { "level": 9, "delegate": "tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx",
    "slots":
     [ 4, 5, 6, 14, 15, 40, 52, 53, 67, 70, 73, 86, 87, 90, 108, 111, 115,
       130, 137, 142, 149, 151, 160, 164, 169, 179, 186, 188, 192, 193, 195,
       197, 199, 202, 216, 217, 238, 242, 245, 252 ],
    "consensus_key": "tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx",
    "companion_key": "tz4W8oYMC6qXQP1fvB7A1oNRgYXUticrZ4Ao" } ]

Manually testing the MR

dune exec tezt/tests/main.exe -- --file companion_key.ml

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