@Override
    public Principal getUserPrincipal() {
        return null;
    }
Opaque token {@link Credentials} usually representing a set of claims, often encrypted
or signed. The JWT (JSON Web Token) is among most widely used tokens used at the time
of writing.

@since 5.3
httpclient5/src/main/java/org/apache/hc/client5/http/auth/BearerToken.java