Allow creating / editing payment trigger "Post Data" input with form fields

[Klakurka]

This would involve parsing the JSON and building a list of inputs to enter values for, and vice versa.

I'm not sure what the best form layout would be for this but we can come up with something.

[Klakurka]

Remember to allow them to add/remove custom KVPs.

[Klakurka]

There's been a high % of "invalid JSON" errors so the goal here would be to have it work kind of like HomeAssistant does with the GUI vs YAML situation, where the GUI is the primary method for setting things up but where they could "go crazy" and just code it themselves (YAML) but it's meant for advanced users who know what they're doing.

[Klakurka]

We should also have auto-detection of a bad entry both when editing and also on initial page load.

I had an issue recently where we removed hmac so my entry was considered invalid even though it was fine before. There was no indication that anything was wrong but it was of course erroring out and that error was logged.