Stop using CryptoTypePublicPair directly

[JoshOrndorff]

When I was first learning the keystore, I fumbled with the types a lot. I ended up getting something that compiled and worked, so I stuck with it. Now it seems more clear that we should be using NimbusIds directly, and calling .into() on them when we really need a CryptoTypePublicPair.

This change needs to be made throughout the nimbus-consensus crate. Especially in the parachain worker and the manual seal ConsensusDataProvider.