Enable platform plugins to prevent logging in if they won't be able to provide authentication

[jmderuty]

Problem

When platform plugins handle authentication for the Users plugin (for instance Steam or EOS), it's possible they won't be able to authenticate the player (because the launcher is not started, the player does not own the game or is not logged in, etc...)

However, right now these situations cannot be handled gracefully before an actual connection is made with the Stormancer authentication server. This is leads to several issues:

• Useless connections to the server.
• More logs than necessary on the client, which makes understanding of the issue much more difficult than would be the case if the connection process was interrupted earlier.

Proposal

• Add a OnLoggingIn event on the IAuthenticationEventHandler contract client side which enable implementations to interrupt the login process early with an associated error message.
• Implement the new event in all platform plugins : device_identifier, STEAM, EOS, GoG, PSN, Nintendo and XBoxLive.

[Falanwe]

Preventing the authentication process from proceding in a OnLogiingIn event would require either

• only detectings states where we know in advance getting the credentials would fail (typically incomplete configuration)
• getting the credentials during the OnLoggingIn method, potentially discovering they can't be retrieved (Steam is closed, the Epic DevAuthTool is not running or not configured, etc...), then hold on to them until retrieveCredentials is called

I propose to instead add two properties to the CredentialContext struct:

• a boolean denoting an unexpected failure
• a string for a custom error message

If the context is marked as fail, the process can stop early and output an appropriate error message