Token endpoint URL used for OIDC cannot be set on the JAAS config

Kafka allows multiple clients within a JVM to use different SASL configurations by configuring the JAAS configuration in `sasl.jaas.config` instead of the JVM-wide system property. For SASL login, we reuse logins within a JVM by caching logins indexed by their sasl.jaas.config. This relies on login configs being overridable using `sasl.jaas.config`.

KIP-768 (https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=186877575) added support for OIDC for SASL/OAUTHBEARER. The token endpoint used to acquire tokens can currently only be configured using the Kafka config `sasl.oauthbearer.token.endpoint.url`. This prevents different clients within a JVM from using different URLs. We need to either provide a way to override the URL within `sasl.jaas.config` or include more of the client configs in the LoginMetadata used as key for cached logins.