Env. Variable Support for JSON request body

[ashitaprasad]

Tell us about the task you want to perform and are unable to do so because the feature is not available

Currently, env variable support available for:

• URL
• URL Params
• Headers

Instead of updating the body every time, it would be great if the support can be extended for JSON request body as well.

[Clasherzz]

@ashitaprasad while implementing this feauture is it necessary to highlight the variable being used .
There is a chance it might conflict with Add syntax highlighting for JSON request body #582 and i feel like its better to implement just variable support .

[ashitaprasad]

@Clasherzz It should work like other places where env variable support is added. If a user hovers over the variable, it should show the value.
It will definitely conflict with #582 so in case a feature gets merged first, the other person must work towards adding on top of it.