Add more variables to THEN automation rules

Issue Description

When we create an automation rule, it would be nice if we could include some other variables besides the ones that are already available in the THEN parameters.

Steps to reproduce:

Create a custom automation rule as follow:
WHEN:

• Issue created

THEN:

• Independent of the parameter chosen. Have the ability to add more variables/custom fields values.

Actual Result:

There are a few hardcoded variables available.

Suggested Solution:

It would be nice if we could add more variables and/or custom fields values to these variables.