Allow system wide flag to enable Instance access to "host" environment variables

### Description

As a FlowFuse Admin
I want to allow Instances to access environment variables from the host environment
So that they can decide on prov/dev or region location

e.g. FlowFuse running on AWS EKS the AWS_REGION env var is injected into pods to allow them to identify their current region

### Which customers would this be available to

Everyone - CE/Starter/Team/Enterprise

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

I have provided an initial effort estimate