Reject VS/VSR resources with enabled plus features for OSS

[bvighnesha]

Proposed changes

Currently, the Ingress Controller for NGINX OSS allows fields in VS/VSR that are only supported in NGINX Plus. For example:

Upstream.HealthCheck
Upstream.SlowStart

If an OSS user starts using a Plus feature, we need to report quickly that the feature is not supported.

Requirements:

If an OSS user configures a health check in a VS/VSR upstream, the IC will reject the resource with a warning event that includes active health checks are only supported in NGINX Plus.

If an OSS user configures slow start for a VS/VSR upstream, the IC will reject the resource with a warning event that includes slow start is only supported in NGINX Plus.

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

[Rulox]

lgtm I just suggest to change the error messages in the Tests to be more consistent with the other messages (also, it is not true that the input is invalid, I think we should remove the invalid). Suggested changes as inline suggestions.

Thanks! I am approving but please have a look at the comments.