add a namespace defaulter to `k8s_yaml`

Describe the Feature You Want

Add the following function to the tiltfile:

k8s_yaml(yaml, namespace='foo')

Tilt should go through all the objects in the YAML and do Helm/kubecontext namespace defaulting:

• If the resource has an explicit namespace, keep the explicit one.
• If the resource does NOT have an explicit namespace, use foo

Current Behavior

If you want namespace defaulting, you have to modify your kubecontext yourself

Why Do You Want This?

Namespace defaulting is a super common feature in the k8s ecosystem.

Additional context
NOTE: this is very different from kubectl --namespace semantics, which are:

• If the resource has an explicit namespace, verify that it's the same, and emit an error if it's not the same.
• If the resource does NOT have an explicit namespace, use foo

but i think those defaulting semantics are wrong for us, and newer tools seem to be moving away from them

Start by locating the k8s_yaml entry point and the existing Kubernetes YAML namespace handling. Check how current tests cover explicit and missing namespaces, then verify that explicit namespaces remain unchanged and missing namespaces receive the supplied default without adopting kubectl's conflicting-namespace behavior.