Make the authenticate method more flexible

[mariocarro]

Is your feature request related to a problem? Please describe.
Some of our FeathersJS servers user several custom fields for the authentication, not only for the username.

Describe the solution you'd like
Just allow (optionally) sending an arbitrary JSON.

Describe alternatives you've considered
Added to the custom username field, we need to be able to use also a custom fieldname for the password, send additional login information (for example: module or facility being accessed), tracking information (that also varies from time to time), etc. A fixed parameter list is too limited. The parameters userName and password should not be required and an optional extraInfo parameter should be added.

Additional context
It's clear that a fixed parameter list is not enough in our case, but also other authentication methods (OAuth, API-key, etc) would not be feasible with the current interface.