Support jwt token auth

[jkroepke]

As more secure alternative to auth basic, add the support via jwt token.

it should possible to define a token file and a header name to attach the token. The token file can be changed each hour.

Use case:

i have to disable the initial admin in grafana. I would like to enable jwt auth to allow login via kubernetes server account token, since its a short-living jwt token.

ref: grafana/grafana#81794