OAuth: Support client_secret_jwt for oauth providers when doing token exchange

**Why is this needed**: Right now when oauth is setup through any provider we always perform code to token exchange using `client_secret_post`. We will do a post request to the token endpoint and always include `client_secret` in the request url. We should implement `client_secret_jwt`. What that means is that we use the client secret to sign a jwt that the IPD also can validate using the shared client secret. More details on it can be found here https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication. We would need to look through our providers and see who can support it. ~It also does not seems like the [oauth2 package we use support it yet](https://github.com/golang/oauth2/issues/744) so either we wait for the pr to be merged or workaround it for now.~ I imagine 1 new config option for providers `client_authentication` that can have two values, either `client_secret_post` (should be default) and `client_secret_jwt`. On top of these we need to update places in the ui where we can do oauth configuration. When doing the exchange we should perform the check on what client authentication method is used in https://github.com/grafana/grafana/blob/main/pkg/login/social/connectors/social_base.go#L92-L97 and handle it correctly.