Add NGINX plus Dockerfile, add make command, add metrics

[ciarams87]

Proposed changes

Problem: To enable support for NGINX Plus we need a way to build NGINX Plus images using a provided license. Additionally, if we are using an NGINX Plus image, we need to update the Prometheus exporter to use the NGINX Plus API instead of the stub_status so we do not get errors when metrics are enabled.

Solution:

• Create an NGINX Plus Dockerfile and conf file
• Create a Make target to build the NGINX Plus image
• Add a build agent arg to the nginx images to indicate if the image was built locally or from the pipeline
• Update the metrics code to use the Plus API instead of the stub status when NGINX Plus is enabled
• Add a new cli flag to indicate that NGINX Plus support should be enabled and add a corresponding entry to the helm templates
• Add documentation.

Testing: Built and deployed the NGINX Plus image as part of my NGF deployment locally and ran the examples

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide
specific feedback, add them here.

Closes #1327
Closes #1330

Checklist

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

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[kate-osborn]

👍