Create emissary-ingress.dev/v4 CRDs

[kflynn]

We have to bump the CRD version anyway, because removing header casing and error response rewriting is a breaking change, and we'd like to be able to get away from apiext for green-field installations of Emissary 4, but we'd also like to not force forklift upgrades from Emissary 3. The way to make all of these things work is to have a new API group.

emissary-ingress.dev/v4 CRDs should mostly look like getambassador.io/v3alpha1, but:

• rip out stuff lingering from v2
• rip out header casing and error response rewriting
• use camelCase instead of snake_case

and yes, we'll all but certainly start with v4alpha1 but let's not stop there.

[kflynn]

Note that we run into the shortname collision problem here, too: does kubectl get Mapping mean mapping.getambassador.io or mapping.emissary-ingress.dev? We should think about that, and how to alleviate it.