Add endpoint discovery to realm

[mingfang]

Keycloak implements the OIDC Discovery specification.
The metadata endpoint is a <base URL>/.well-known/openid-configuration.
This endpoint returns all the other endpoints that are needed by applications, like this one
https://github.com/mrparkers/terraform-provider-keycloak/blob/master/example/external_token_exchange_example.tf#L27

With the new endpoint discovery feature, that code may look like this

  authorization_url  = keycloak_realm.token-exchange_source_realm.authorization_url

or like this, if we want to put all the endpoints in one place

  authorization_url  = keycloak_realm.token-exchange_source_realm.endpoints.authorization_url