feat: Terraform: Add support for shared VPC

[manitgupta]

• Adds support for launching and configuring a shared VPC.
• Adds support for configuring a shared VPC in migration templates.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 48.38%. Comparing base (3584474) to head (2f48244). Report is 30 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1723 +/- ## ============================================ + Coverage 42.20% 48.38% +6.18% + Complexity 3159 992 -2167 ============================================ Files 790 327 -463 Lines 45897 17685 -28212 Branches 4903 1767 -3136 ============================================ - Hits 19369 8557 -10812 + Misses 24951 8539 -16412 + Partials 1577 589 -988 ``` | [Components](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1723/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [spanner-templates](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1723/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `63.12% <ø> (-0.38%)` | :arrow_down: | | [spanner-import-export](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1723/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `∅ <ø> (∅)` | | | [spanner-live-forward-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1723/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `74.97% <ø> (+0.83%)` | :arrow_up: | | [spanner-live-reverse-replication](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1723/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `51.44% <ø> (+0.87%)` | :arrow_up: | | [spanner-bulk-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1723/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `83.14% <ø> (-0.32%)` | :arrow_down: | [see 503 files with indirect coverage changes](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1723/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform)

[manitgupta]

Responded to the comments. The current folder structure already signifies the value of these templates, from the README.md of v2/spanner-common -

This repository provides samples for scenarios that emulate common customer scenarios. These are especially useful is setting up test infrastructure that resembles a customers' environment

https://github.com/GoogleCloudPlatform/DataflowTemplates/blob/main/v2/spanner-common/terraform/samples/README.md#terraform-samples-for-common-spanner-setups