@Override
  public void apply(RequestTemplate template) {
    template.header("Authorization", headerValue);
  }
Creates an interceptor that authenticates all requests with the specified username and password
encoded using the specified charset.

@param username the username to use for authentication
@param password the password to use for authentication
@param charset the charset to use when encoding the credentials
core/src/main/java/feign/auth/BasicAuthRequestInterceptor.java