Split out Azure Firewall, Gitea and Nexus from core TRE components

[tanya-borisova]

Description

As a customer of Azure TRE
I want to opt-in into having {Azure Firewall, Gitea, Nexus} deployed
So that I can decide whether it meets my needs

Acceptance criteria

• Firewall deployment is decoupled from the deployment of the core TRE components
• Gitea deployment is decoupled from the deployment of the core TRE components
• Nexus deployment is decoupled from the deployment of the core TRE components

Notes

This is related (and likely part of) issue #23
This is also related to resolving bug #1165 (as implementing this will likely make the bug go away)

[tanya-borisova]

It turns out that Azure Firewall currently can't be destroyed from Terraform (but can from Portal).
Link to the issue in the azurerm Terraform provider: hashicorp/terraform-provider-azurerm#14866

To deploy this change, we would have to ask everybody to manually delete Firewall in their dev deployment, as well as delete it manually for the CI deployment.

[tanya-borisova]

It also turns out that Gitea and Nexus won't work as part of core terraform if Firewall is extracted into the shared service, so all 3 components are extracted into Porter bundles as part of this ticket

[tanya-borisova]

Talked to @marrobi today and we agreed that in order to minimise disruption to existing users of TRE, I will attempt to run terraform state rm for the resources in core terraform, followed by terraform import in the Porter bundles terraform states.
This is to ensure continuity of existing deployments of Firewall, Gitea and Nexus, as terraform won't destroy and create them again.