BEEFY: define on-chain beefy-genesis and use it to coordinate voter initialization (#13215)

* beefy: add support to configure BEEFY genesis

* client/beefy: more flexible test runtime api

* client/beefy: add tests for custom BEEFY genesis

* client/beefy: ignore old state that didn't account for pallet genesis

* client/beefy: fix clippy

* frame/beefy: default BEEFY-genesis is block One::one()

* frame/beefy: add extra doc comments

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Co-authored-by: parity-processbot <>