Arbitrary k8s readiness checks

From @jfancher:

is there any workaround for, or plan to support, depending on readiness of arbitrary k8s objects? For example... I have a KafkaTopic CRD managed by an operator [which is its own resource b/c of a call to k8s_kind]; it obviously has no pods behind it as it's just doing some configuration, but it does have a readiness condition. It'd be nice to make apps that depend on it wait; also, it's moderately annoying that that resource never turns green in Tilt

This feature (in conjunction with #2989) could also help ClusterAPI; see e.g. this hack around readiness conditions:

local("kubectl apply -f https://github.com/jetstack/cert-manager/releases/download/{}/cert-manager.yaml".format(version))

# wait for the service to become available
local("kubectl wait --for=condition=Available --timeout=300s apiservice v1beta1.webhook.cert-manager.io")

I.e. all resources need to wait on on this YAML being applied and the resources being ready. If Tilt supported arbitrary k8s readiness checks, this YAML could be its own resource, and only go green (i.e. only trigger its deps to build) when the wait condition was met.

Start by reading the k8s_kind entry point and the linked ClusterAPI Tiltfile workaround. Trace how Kubernetes resources become ready and how readiness gates dependent builds. Done means arbitrary resource readiness conditions, such as KafkaTopic or APIService conditions, can control resource status and dependency triggering.