[feature request] Add an ability to pass variables to Parse Job API endpoint

Hi!

Nomad has very useful [parse endpoint](https://www.nomadproject.io/api-docs/jobs#parse-job)

What we're missing is the ability to pass variables, like you can do using a nomad CLI, to override variables before parsing and templating.

I see it as adding an optional argument `Variables` as a hash with key\values where each key could represent the name of the variable to override.

Currently, the only option is to use CLI for that.

Start with the Parse Job API documentation and compare it with the Nomad CLI's variable override behavior. Trace the parse and templating entry points, then verify that an optional Variables hash can override values before parsing and templating through API-level tests.

Written by the indexing model from the issue text.