Proto: Allow implicit accounts to delegate at bootstrap time

Context

Implements #1972 (closed). I considered adding an additional entry to the lists in bootstrap_accounts (which are presently [account, tez]), but dealing with the existing two-element lists was pretty complicated, so I decided to do it this way.

The new parameter is:

bootstrap_delegates: [{
"from_pkh" : "tz1...",
"to_pkh" : "tz1..." },
...
}

Manually testing the MR

Manually tested in tezos-k8s, and, after some hackery, works. Will file a tezos-k8s MR for the necessary changes.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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