Refactor/organize the whole project, e.g. move participant and coordinator to inside frost-client. Possibly remove their old CLI interface?

[mpguerra]

No description provided.

[conradoplg]

We should also improve the usage of frost-client as a library, making sure the pubic API makes sense and improving error handling for the public API.

[conradoplg]

Also desirable

• Refactor out the challenge/login flow out of participant (http.rs), coordinator (http.rs), and frost-client (session.rs) into a single function
• Create a client object to query the API, so that it's not required to use raw reqwest everywhere (though that may be more useful as an example?)