lib/babe: implement secondary slot block production

[noot]

Task summary

• gossamer already has secondary slot verification, we can use this to implement secondary slot production
• if the ConfigData for an epoch has secondary = 1 or 2 we should enable secondary slot production
• this can be done by calling getSecondarySlotAuthor for each slot in the epoch and determining if we can author a block in that slot (similar to the existing lottery)
• we will also need to update pre-runtime digest when producing a block in secondary to be of type BabeSecondaryPlainPreDigest
• also, when is BabeSecondaryVRFPreDigest used? (needs info from spec team probably)
• add stress tests as well

Specification reference

Other information and links

• https://research.web3.foundation/en/latest/polkadot/block-production/Babe.html
• https://polkadot.network/blog/polkadot-consensus-part-3-babe/