Add signup api

[mederic-p]

Closes #77

Overview

This PR adds the new client signup API.

Changes

• Add SignupParams
• Add signup endpoint

Usage

The new endpoint can be used as follow:

let credentials = ClientCredential(apiKey: "api_key")
let config = ClientConfiguration(baseURL: "http://localhost:4000", credentials: credentials)
let client = HTTPClientAPI(config: config)
let params = SignupParams(email: "email@example.com", password: "password", passwordConfirmation: "password")
client.signup(withParams: params) { (result) in
    switch result {
    case .success: break
    case let .fail(error: error): break
    }
}