Add Molecule tests covering common NGINX configurations

[alessfg]

Proposed changes

The current Molecule tests dedicated to NGINX configurations are limited to testing all NGINX Open Source and NGINX Plus directives. It works, but it is very unhelpful in helping users figure out how to use this role. By adding more tests covering specific NGINX use cases, we can point users to the working playbooks as a guide when implementing this role in their workflows. This PR includes new tests for configuring NGINX as a:

• Web server
• Reverse Proxy
• Configure the NGINX stub status metrics
• Configure the NGINX Plus API and live metrics dashboard

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the CONTRIBUTING document.
• I have added Molecule tests that prove my fix is effective or that my feature works.
• I have checked that any relevant Molecule tests pass after adding my changes.
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md).