allow to use json file as is without replacing/parsing anything

[talgendler]

Summary

Allows to treat payload within a file unchanged, no replacing {{ similar to how one would specify payload with all the slack blocks etc but using template strings to be parsed by github itself.

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.