[auth.jwt]: Configure multiple JWT auth providers (or JWKS URLs)

**Why is this needed**: In some situations, the same Grafana instance might need to be accessed via different JWT auth providers; however, currently, only one JWKS URL can be configured, as such, a JWT token from one of the providers won't be valid. **What would you like to be added**: Option to configure multiple `auth.jwt` sections, or multiple `jwk_set_url`s. **Related issues**: - https://github.com/grafana/grafana/issues/109373