Add admin login API

[mederic-p]

Closes #81

Overview

This PR adds the admin login endpoint.

Usage

Use is very similar to the client login endpoint:

let config = AdminConfiguration(baseURL: "http://localhost:4000")
let apiClient = HTTPAdminAPI(config: config)
let params = LoginParams(email: "admin@example.com", password: "password")
apiClient.login(withParams: params) { (response) in
    switch response {
    case .success(data: let data): print(data)
    case .fail(error: let error): print(error)
    }
}