Additional parameters for authenticate_user

[racke]

Unfortunately we restrict the parameters passed to authenticate_user to username and password.

https://github.com/PerlDancer/Dancer2-Plugin-Auth-Extensible/blob/master/lib/Dancer2/Plugin/Auth/Extensible/Provider/Config.pm#L75

It would be very useful to have additional parameters, especially if you want to authenticate
to a database selected by the user (as with a database admin interface).

This just came up again in a discussion on #ledgersmb.

[ehuelsmann]

Do you want a PR for this? I'm from ledgersmb/LedgerSMB and I'd like to start using Dancer2 with this plugin.

[racke]

Yes please 😎 . If you need assistance let us know.