Add VPC and Cloud Run v2 API support to the Cloud Run App Terraform module

The module will currently create a VPC for the purpose of assigning a static external IP to the Cloud Run service, but it can't assign the Cloud Run service to an existing VPC for ingress protection purposes. This issue covers adding support for specifying an existing VPC (as a variable) and connecting the Cloud Run service to that VPC such that ingress traffic will only come from the VPC ("internal"). This should include setting up the connection to the VPC (either Serverless VPC Access connector or Direct VPC), and permitting the use of Private Service Connect (I don't think any changes are required to this module for that, but needs to be confirmed).

Using a VPC appears more straightforward with the Cloud Run v2 API, so the option to use Cloud Run v2 API should be supported as part of this change (and using v2 should be compulsory when using an existing VPC).