[#811] feat: address as string and remove redundant opreturn

[chedieck]

Related to #811

Description

Some minor fixes to SimplifiedTransaction:

• paymentId is made mandatory, as is message. It will just be empty if it doesn't exist.
• redundant opReturn removed
• address is sent as a string, not full address object.

Test plan

Build the client using PayButton/paybutton#387. Test if:

• Triggers execute without erroring,
• The client receives a tx and executes onSuccess /onTransaction without erroring.