Provide integration with external authentication systems for the RUM endpoint

In some use-cases (such as [this one](https://github.com/elastic/apm-agent-js-base/issues/136)) the RUM endpoint could be secured using the authentication system already in place for the web application that is being monitored. This would mean that we only accept payloads with a valid authentication token (therefore only for logged-in users).

Furthermore, we need to investigate the scope of authentication systems for which we can provide support. But, we can start with supporting `Authorization: Bearer ` header with JWT.

This feature should be optional and disabled by default.