Mithril Signer retrieve pending certificate

[jpraynaud]

The Mithril Signer needs to retrieve the certificate pending from the Mithril Aggregator:
(only for one route: /certificate-pending, the other routes will be in separate task)

• Add an AggregatorHandler trait that will be responsible for communications with the aggregator (as done in the client)
• Add an AggregatorHTTPClient that implements this trait
• Setup DI for this trait