code: func (c *Client) RotateExternalCertAuthority(ca services.CertAuthority) error {
  	if err := ca.Check(); err != nil {
  		return trace.Wrap(err)
  	}
  	data, err := services.GetCertAuthorityMarshaler().MarshalCertAuthority(ca)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  	_, err = c.PostJSON(c.Endpoint("authorities", string(ca.GetType()), "rotate", "external"),
  		&rotateExternalCertAuthorityRawReq{CA: data})
  	return trace.Wrap(err)
  }
docs: // RotateExternalCertAuthority rotates external certificate authority,
  // this method is used to update only public keys and certificates of the
  // the certificate authorities of trusted clusters.