Allow the 'KeycloakAdmin' class to instantiate against non-master realms using
an Authorization header for a non-admin user that is granted permissions to
inspect or manage Keycloak admin resources.
Example:
kca = KeycloakAdmin(
'https://auth.keycloak.local/auth/',
realm_name='my-realm',
client_id='admin',
custom_headers=auth_headers
)