Block deploying to device if required env vars are missing

### Epic

_No response_

### Description

**As a:** user

**I want to:** block a snapshot being deployed to a device if the device is missing required env vars

**So that:** I do not deploy broken flows

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Currently as soon as a device is assigned to a project, the current snapshot will be deployed to it. However, if the snapshot requires some device-specific configuration to be provided via env vars, there is a problem - that initial deploy of the snapshot won't have the necessary env vars set.

One workaround is for the user to define the env vars *before* assigning it to the project - but that is somewhat counter intuitive. See also #937 - a consistent UX should be used around showing a list of known env vars from the flows to help the user set them - but that would only work if the device has been assigned to the project so we know what env vars exist.

We don't have the concept of a 'required' env var - and I'm not sure we necessarily want to introduce one. But we do need a way to allow a user to set project-specific env vars on a device *before* the snapshot is deployed.

### Which customers would this be availble to

All Users, (CE)

### Acceptance Criteria

_No response_