Implement a common aggregator client - Phase 2

[jpraynaud]

Why

Following the issue #2640 where we have implemented the first phase of the common aggregator client, we want to implement the second phase which consists in using the common aggregator client in the client library.

What

Use the new common aggregator client in the client library and remove the previous implementation of the aggregator client.

How

• Implement all the HTTP clients in the Mithril client library with requests in the new common aggregator client
• Remove the AggregatorClient trait and replace it with AggregatorRequest traits (one for each sub-client)
• Create a trait AggregatorClient that can be injected in the client builder to replace the implementation of the communication with an aggregator (Do we do that now?)
• Remove the deprecated with_aggregator_client function in the Mithril client library
• Publish manually the first version of the common aggregator client crate on crates.io + setup crate owner (@jpraynaud)