Apply Configuration of SnippetsFilter to NGINX

[mpstefan]

As a user of NGF
I want my SnippetsFilter configuration applied to NGF's data plane
So that I can leverage NGINX features not yet available in NGF.

Acceptance

• The SnippetsFilters is validated to ensure it only contains valid contexts:
  • main
  • http
  • server
  • location
• The SnippetsFilter only allows one snippet per context.
• If a resolvedRef of a SnippetsFilter is found to be invalid for a given Route rule, that Route rule is configured to return a service unavailable response that is appropriate for the route.
• Configuration defined in a filter attached to a route rule applies the NGINX directive configuration specified in the filter.
• SnippetsFilters are disabled by default
• SnippetsFilters must be explicitly enabled.

### Tasks
- [x] Register SnippetsFilter controller if feature is enabled
- [x] Validate SnippetsFilters add add to the graph
- [ ] Apply SnippetsFilter to NGINX config