core/consensus,node/runtime: Declare and implement authorities endpoint (#3207)

The goal of the commit is to be able to retrieve the current set of
authorities without needing to know the concrete consensus mechanism in
place.

In order to achieve the above this commit introduces the
`core/consensus/common/primitives` crate, declaring the `ConsensusApi`
runtime API. In addition it implements the above mentioned trait
definition in `node/runtime` by returning the current authorities of the
BABE consensus mechanism.