Feature: use variables in body from external file

[countcain]

Issues

• I have checked existing issues and there are no existing ones with the same request.

Feature description

Let's say we have a http file

### The request body is read from a file
POST https://example.com:8080/api/html/post
# rest.nvim can guess the Content-Type from file path

< ./input.json

variables in the external file input.json don't get replaced with values.

[countcain]

nice! thank you very much!