Add VirtualServer/VirtualServerRoute snippets support

[Dean-Coakley]

Proposed changes

• Add http-snippets field to VirtualServer to allow adding NGINX config snippets to the http context.
• Add server-snippets field to VirtualServer to allow adding NGINX config snippets to the server context.
• Add location-snippets field to VirtualServer and VirtualServerRoute to allow adding NGINX config snippets to the location context.
• Add enableSnippets cli arg.

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 master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[LorcanMcVeigh]

LGTM