testnets: configure delegations at genesis

Pre-Babylon, Tezos had delegation contracts, but it was then changed to give a native ability for a tz* address to delegate to another tz1 address.

When activating a new protocol (for example for testnet bootstrap), I can pre-populate the chain with a KT1 delegation contract, but I can not pre-configure tz addresses to delegate to one another from the beginning.

That would be very useful for example to test TRD (tezos reward distributor) in isolation in a private chain. It would have no effect on mainnet though (which is probably why no one has worked on this yet).

This would require a change in the proto-proposal.