This PR introduces the pallet for Sassafras consensus.

## Non Goals

The pallet delivers only the bare-bones and doesn't deliver support for
auxiliary functionalities such as equivocation report and support for
epoch change via session pallet.

These functionalities were drafted in the [main
PR](https://github.com/paritytech/polkadot-sdk/pull/1336), but IMO is
better to introduce this auxiliary stuff in a follow up PR and after
client code.

## Potential follow ups

https://github.com/paritytech/polkadot-sdk/issues/2364

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Co-authored-by: Sebastian Kunert <[email protected]>
Co-authored-by: Koute <[email protected]>
Co-authored-by: Michal Kucharczyk <[email protected]>
Co-authored-by: André Silva <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>