split k8s_custom_deploy into multiple resources

Current Behavior

Currently, when you deploy a Helm chart with helm_resource, Tilt tracks it as a single resource. This means:

• there's one KubernetesDiscovery object
• there's one UIResource

This means that if your chart has multiple deployments in the chart:

• there's only one item in the UI
• all deployments log to the same resource
• restarting the item redeploys the whole chart
• tilt picks one pod to determine health check status / readiness
• tilt picks one pod to do port forwarding

Describe the Feature You Want

A way to represent the chart as multiple resources in the Tilt UI

Additional context
I have mixed feelings about this. We may have to break it up into multiple feature requests. The problem is that a "resource" in tilt aggregates many different things, and breaking them up might get really hairy (e.g., if you break a chart into 2 resources, and restart one, does that mean the other one must restart? If the answer is "no", that breaks Helm chart install semantics. If the answer is "yes", that breaks Tilt restart semantics.)

Start by tracing helm_resource and how it creates the single KubernetesDiscovery and UIResource described in the issue. Define how multiple chart resources would affect UI grouping, logs, health checks, port forwarding, and restart behavior, while preserving Helm install semantics; the issue notes these tradeoffs are unresolved.