Add resource to each namespace

[mamachanko]

Describe the problem/challenge you have
Specifically, I would like to add a resource to all namespace. For that I must be able to set the resources .metadata.name, but its not known.

What I have so far:

#@overlay/match by=overlay.subset({"kind": "Namespace"}), expects="0+"
#@overlay/insert after=True
---
kind: Thing
metadata:
  name: the-thing
  namespace: ?

To be more specific
given

#! stuffs.yaml
---
apiVersion: v1
kind: Namespace
metadata:
  name: one
---
apiVersion: v1
kind: Namespace
metadata:
  name: two

want

ytt -f stuffs.yaml -f magic.yaml

---
apiVersion: v1
kind: Namespace
metadata:
  name: one
---
kind: Thing
metadata:
  name: thing
  namespace: one
---
apiVersion: v1
kind: Namespace
metadata:
  name: two
---
kind: Thing
metadata:
  name: thing
  namespace: two

Describe the solution you'd like

Of course, there's a general pattern here, but I am expressing this as the specific use case mentioned ^.

Anything else you would like to add:
According to our Slack conversation, this is not easily done yet.

---

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[pivotaljohn]

Note: the solution to this is likely to also resolve #718

[pivotaljohn]

We've seen multiple situations where a user would love to accept an entire document as an input into a template that results in a new document. Since this is a mere extension of the existing semantics of overlays (i.e. implementing what it means to insert or append a document), this seems like a fine addition the the feature set.