Signals for the external acquisition of an api_access_info

[Dimfred]

With this PR I would like to introduce two new signals, to enable the api login process through an external_service.

Signal1: application::login_attempt emitted at the end of application_impl::new_connection.

Signature:     
bool& // should be set by the external service to true, if it is active. 
      // It's bool& to avoid return types 
      // cause this would result in returning a boost::optional
string& // username
string& // password
login_api& // the created login_api (needed to connect the second signal)

Signal2: login_api::api_access_info_external emitted at the beginning of login_api::login.

Signature:
string& // username
fc::optional<api_access_info>& // to be filled by the `external_service`

Logic:

signal::login_attempt is emitted
    => if no service connected: default login
        => signal::api_access_info_external emitted
           => does not set the optional 
           => from here default login
    => else: rerouted login through the external_service
       => service connects to login_api::signal::api_access_info_external           
          => singal::api_access_info_external emitted
             => service trys to find api_access_info to given user
                => if found: login with found
                => else default login

Alternatively this could be done with the first signal alone, but then the service would have to implement the login logic, which I tried to avoid here.

What do you think? Would this be a way to implement that? Open for any discussion.

[pmconrad]

I'm not really clear which problem you are trying to solve here.
Please open an issue describing the problem first, then we'll discuss how to best solve it.

[abitmore]

At the moment, I do think this is interesting. Unfortunately, the code is incomplete - no test cases or example code to show how to interact with it.

[Dimfred]

Oh this was quite a while ago. If I find time I could look at what I even did there and maybe add tests / an example on how it could be used.