Require environment variables and provide validation

### Description

Description: Currently, when deploying a Node-RED instance on FlowFuse, there is no validation to ensure that all required environment variables are present. This can lead to issues where critical variables like passwords are missing, causing the instance to fail to start properly.

Proposed Feature: Allow users to mark environment variables as "required" when defining them in the FlowFuse template.

When deploying an instance, the platform should validate that all required environment variables have a non-blank value set. If any required variables are missing, provide a clear alert or error message to the user before allowing the deployment to proceed.

This validation could happen at multiple points, such as:

In the deployment UI, showing a warning if any required vars are missing
In the DevOps pipeline, failing the deployment if required vars are not present

Benefit: This feature would help ensure that critical environment variables are always properly configured before deploying a Node-RED instance, preventing runtime issues due to missing values.

### Which customers would this be available to

Enterprise Tier Only (EE)

### Have you provided an initial effort estimate for this issue?

I can not provide an initial effort estimate