Common EngineSigner

[keorn]

Reusable EngineSigner for signing consensus messages. Goes after #4184.

[coveralls]

Changes Unknown when pulling 20114c8 on engine-signer into ** on master**.

[rphmeier]

Documentation on all public items would be a plus

[rphmeier]

Why not take it a step further and make EngineSigner a trait with its current interface? Would serve to decouple Engine from implementation details like AccountProvider and make it more testable via mocked signers.

[keorn]

Its a very simple struct and is tested via multiple Engine tests. I dont see much reason for different implementations for now, once we need them it can be easily turned into a trait. imho this is simpler

[gavofyork]

need resolving.

[coveralls]

Changes Unknown when pulling 1ae411d on engine-signer into ** on master**.

[gavofyork]

i was also imagining a trait + trait object, but this is a decent first step.