feat!: split network step

[mauro-cit]

Fixes #672
Fixes #671

Some notes about the changes done by this PR:

• As we have separated the step 3-networks, the guide for doing that was through the enable_hub_and_spoke variable.
• Basically the resources that required the variable enable_hub_and_spoke as true we put on 3-networks-hub-and-spoke, the resources that doesn't we put on 3-networks-dual-svpc. We also moved the resources that were depending on these ones.
• We also removed the variable enable_hub_and_spoke_transitivity from the Dual Shared VPC mode since it was only used when the enable_hub_and_spoke variable was true
• A script called restore_tf_files.sh was also created to undo the actions that the "disable_tf_files.sh" was doing to the *.tfvars and backend files

[daniel-cit]

The Hub & Spoke Transitivity module

https://github.com/mauro-cit/terraform-example-foundation/tree/feature/split-network-step/3-networks-dual-svpc/modules/transitivity

should be removed from the dual svpc version

there are also some fixes on the readme and shared.auto.example.tfvars files in the Hub and Spoke version

[bharathkkb]

Thanks for the PR @mauro-cit
We can also remove some of the conditional logic we no longer need like in

terraform-example-foundation/3-networks-dual-svpc/modules/base_shared_vpc/variables.tf

Line 27 in 997d699

variable "mode" {

[bharathkkb]

LGTM with a suggestion