[#774] feat: op return variable on triggers

[chedieck]

Related to #774

Depends on

• [#774] feat: basic OP_RETURN parsing #782

Description

Adds as a valid variable for setting up triggers.

Test plan

1. Create your trigger, use a website you control or a dummy one such as https://httpbin.org/post.
2. Make sure to include the variable in the trigger body, such as {"op": <opReturn> }
3. Send a tx to the address with OP_RETURN=paybutton$anyDataYouWant and make sure it appears in the receiving end of the POST request e.g. {"op": 'anyDataYouWant'}

[Klakurka]

Really cool!