Add CLI support for getting and setting Environment Group variables

[mauruiz20]

We use Render Environment Groups to manage environment variables shared across services, and it would be very useful to be able to both retrieve and update these variables directly through the Render CLI.

A workflow similar to Heroku’s config:get and config:set commands would make it much easier to inspect and manage remote environment configuration from local development and automated scripts.

Ideally, the CLI could support commands along the lines of:

# Retrieve variables
render env-group get <environment-group>

# Set or update a variable
render env-group set <environment-group> KEY=value

The exact command syntax isn’t important, but having CLI support for both reading and updating Environment Group variables would be very helpful.

Thanks!