Proto: allow to register a governance dictator for testnets and private chain (part 1/2)

This is implementation for tzip!178 (closed)

Compared to the TZIP, it is feature complete but lacks migration code for the ithacanet testnet, the migration code can be found in the followup MR !5612 (merged).

There a minimal automatic test:

dune exec tezt/tests/main.exe -- --color --info --file governance_dictator.ml

And some manual tests: https://gist.github.com/nicolasochem/28eed695ec9b20bf45dd8a0395477ef3