JWT Auth: Allow insecure JWKS URLs

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: This is a bit of a duplicate of #40761, but hopefully with more clear parameters. I'd like to enable JWT authentication and my JWKS URL is within the same private network as Grafana with local dns using http. I'd like to do this without changing `app_mode` from `production` to `development`. Keys may be rotated, so using a `jwk_set_file` is not an option. **What would you like to be added**: Add an explicit option to allow the `http` scheme for `auth.jwt.jwk_set_url`. This value would be off by default, requiring opt in. Example: ``` [auth.jwt] enabled = true jwk_set_url = http://example.xyz/path/to/key/set allow_insecure_jwks_url = true ``` **Who is this feature for?** Uses cases where JWT authentication is desired and the token provider is not external to the private network that Grafana is hosted in, like VPCs, K8s clusters, docker networks, etc.