Move ingress handling to a resource processor

[impl]

Description

This change moves the handling of IngressClasses and Ingresses to their own resource processors. Because this was the last Kubernetes type being handled by the fetcher, we also remove the legacy code for handling different types. Going forward, all new Kubernetes object handling will go into a separate processor type.

This change depends on #3156.

Testing

This change passes the automated test suite. No behavior is changing.

Tasks That Must Be Done

• Did you update CHANGELOG.md?
  • Is this a new feature?
  • Are there any non-backward-compatible changes?
  • Did the envoy version change?
  • Are there any deprecations?
  • Will the changes impact how ambassador performs at scale?
    • Might an existing production deployment need to change:
      • memory limits?
      • cpu limits?
      • replica counts?
    • Does the change impact data-plane latency/scalability?
• Is your change adequately tested?
  • Was their sufficient test coverage for the area changed?
  • Do the existing tests capture the requirements for the area changed?
  • Is the bulk of your code covered by unit tests?
  • Does at least one end-to-end test cover the integration points your change depends on?
• Did you update documentation?
• Were there any special dev tricks you had to use to work on this code efficiently?
  • Did you add them to DEVELOPING.md?
• Is this a build change?
  • If so, did you test on both Mac and Linux?

[kflynn]

Given that you ran the Ingress tests offline and they passed, I'm'a take this. Thanks!! 🙂