OpenID: backchannel_authentication_endpoint

**Is your feature request related to a problem? Please describe.**
Would it be hard to integrate openID: backchannel_authentication_endpoint

https://openid.net/specs/openid-client-initiated-backchannel-authentication-core-1_0.html

It looks like its not supported by now:

```
{
"issuer": "https://sso.example.com/application/o/radioauth/",
"authorization_endpoint": "https://sso.example.com/application/o/authorize/",
"token_endpoint": "https://sso.example.com/application/o/token/",
"userinfo_endpoint": "https://sso.example.com/application/o/userinfo/",
"end_session_endpoint": "https://sso.example.com/if/session-end/radioauth/",
"introspection_endpoint": "https://sso.example.com/application/o/introspect/",
"response_types_supported": [
"code",
"id_token",
"id_token token",
"code token",
"code id_token",
"code id_token token"
],
"jwks_uri": "https://sso.example.com/application/o/radioauth/jwks/",
"grant_types_supported": [
"authorization_code",
"refresh_token",
"implicit"
],
"id_token_signing_alg_values_supported": [
"HS256"
],
"subject_types_supported": [
"public"
],
"token_endpoint_auth_methods_supported": [
"client_secret_post",
"client_secret_basic"
],
"acr_values_supported": [
"goauthentik.io/providers/oauth2/default"
],
"scopes_supported": [
"profile",
"openid",
"email"
],
"request_parameter_supported": false,
"claims_supported": [
"sub"
],
"claims_parameter_supported": false
}
```

**Describe the solution you'd like**

Since Radius is not integrated by now It might be a way to integrate radius and vpn client support like seen here:

https://github.com/okzk/oidc-radius
https://github.com/kurochan/oidc-wireguard-vpn

Thank you.