Implement a publisher abstraction for publication of signatures in signer

[jpraynaud]

Why

We want to create a "publisher" abstraction for the publication of the signatures in the signer to support multiple implementations. In particular, we need to be able to support an implementation for the DMQ node and for the dual publication with the aggregator client as a fallback during the DMQ ramp up phase.

What

Create a SignaturePublisher trait and an implementation with the aggregator client.

How

• Create a SignaturePublisher trait to publish a signature
• Implement a HTTPSignaturePublisher which uses the aggregator client to send the signature on the aggregator REST API
• Implement dependency injection to support the SignaturePublisher trait