feat: add standalone multi-project example

[henrybell]

This PR adds a standalone multi-project example that configures a CI/CD pipeline across multiple projects:

• A central management project to house the CI/CD pipeline and related components, and
• Three "target" projects, each with a GKE runtime environment: dev, qa, and prod.

This example more closely emulates "best practices" in terms of recommendations for project-level separation.

Note:

• The Terratest code is largely duplicated from the standalone single-project example, with minor changes. These could be combined from a DRY perspective, but would no longer be fully standalone.
• Naming is more abbreviated than ideal in some places because of length limits in certain fields. project_id is an example of this.