Refactor offchain api again

[dvc94ch]

The AuthorityKeyProvider trait should be split. Aura/Baba/Grandpa should implement that trait individually.

Open questions:
How to register authority key providers
How to dispatch to an authority key providers through the offchain worker api

[folsen]

Should be fixed with #3213 once that's merged.

[gavofyork]

Yup - we don't want any kinds of "authority key providers". They should look after their own business and not be seeking to fulfil any normative PoS API.