addding JWS Header
adding JWS Payload for the first time
NOTE: This field will be ignored when this is 2nd time and later since existing payload will be used.
NOTE: You can press 'Append Signature' one or more times to add
multiple parallel signatures to a JWS-JS. If you want to change
JWS Header and signing key for each signatures, please modify
above 'JWS Header' and 'signing key' fields then push 'Append Signature'.