Support "plan" mode deployments so that you can evaluate the terraform plan before deciding to apply it.

Upgrading a TRE between versions may change a large number of Azure resources which introduces risk to a running TRE. Being able to evaluate the terraform plan before applying would reduce the risk of issues occuring during an upgrade, and the downtime required for an upgrade.

Ideally this would be a new build variable, e.g.

Also: a similar mode when upgrading components within the TRE.