code: func withConfigSignature(r io.Reader) RequestOption { // nolint
  	return func(ctx context.Client, opts *requestOptions) error {
  		return createConfigSignatureOption(r, opts)
  	}
  }
docs: // withConfigSignature allows to provide a pre defined signature reader for resmgmt client's SaveChannel call
  //  The r reader must provide marshaled ConfigSignature content built using either one of the following calls:
  // * CreateConfigSignature call for a signature created internally by the SDK
  // * CreateConfigSignatureData call with signingBytes used for creating a signature by external tool (ex: Openssl)
  //
  // Note: call this function for as many times as there are signatures required for the channel update.
  // This option appends 1 ConfigSignature read from r to requestOptions.Signatures.
  //
  // Note : function not exported for now TODO: double check how to export this