generate code for caches

[ilackarms]

currently we have a lot of duplicated (kubernetes) caching code across our projects.

ideally we should be able to generate these as they are mostly copy-paste of each other

examples:

https://github.com/solo-io/solo-kit/blob/master/pkg/api/v1/clients/kube/cache/cache.go
https://github.com/solo-io/gloo/blob/master/projects/knative/pkg/api/custom/knative/cache.go
https://github.com/solo-io/gloo/blob/master/projects/clusteringress/pkg/api/custom/knative/cache.go
https://github.com/solo-io/supergloo/blob/master/pkg/api/custom/clients/smi/cache.go
https://github.com/solo-io/supergloo/blob/master/pkg/api/custom/clients/linkerd/cache.go

the generation need not be done by solo-kit itself. this problem is (currently) kubernetes-related and does not deal with solo-kit resources. however, these caches are consumed by solo-kit clients, so it may make sense to handle as part of solo-kit itslef

[ilackarms]

@rickducott @yuval-k @marcogschmidt @EItanya