add generic oauth login

[mistadave]

changes

• generic oauth login
• flag for oauth_auto_login true|false
• variable for input field username of generic oauth provider
• variable for input field password of generic oauth provider
• added sample bash command in doc (url wan't work out of the box, since i used a local instance to test it)

made a own category in the config yaml, that it's not mixed up with the gcom or local login methods

For testing, i used Keycloak 10.0.1 as oauth provider with Grafana 7.3.2.

[klyushkov]

@briangann Hi, can I help you somehow? I'm trying to migrate from local auth to OAuth provider and unfortunately can't do it because don't want to lose grafana-kiosk support.

Merging this PR will be a big step to add other OAuth providers, e.g. Gitlab.

[briangann]

sorry for the delay, i'll test this out