Support diffs in terraform plan

[braunsonm]

Describe the problem/challenge you have
This is an issue to bring back the functionality to show diffs during the plan phase. This does seem possible as the Helm provider does the same thing here:
hashicorp/terraform-provider-helm#702

Describe the solution you'd like
A solution similar to the helm provider that shows the diff in the plan for deploying

Anything else you would like to add:
This used to be supported but caused a lot of issues where the kapp "age" would trigger terraform to crash due to the diff changing between the plan and apply phases. This would need extensive testing to not reintroduce this issue.

[marcinkubica]

Diffing also doesn't happen for apply.