[P037] Add variable processing for JSON attributes

[tonhuisman]

Resolves #5632

Features:

• [P037] Process variables in the JSON Attribute field to allow for longer attribute names (input is limited to 20 characters)
• Update documentation

TODO:

• Testing
• Other similar adjustments?

[tonhuisman]

This allows to set a variable in Rules, f.e. in the On System#Boot Do event, like LetStr,,timeLeft,printer_data.progress.printTimeLeft, and use that variable in the JSON Attribute field: [str#timeLeft]