Add a handy matcher that allows to check for just key existence

**Describe the problem/challenge you have**
I want to create an overlay that identifies an array entry by its key and appends to that array entry. This occurs in kapp-controller with the following:

```
apiVersion: kappctrl.k14s.io/v1alpha1
kind: App
metadata:
name: simple-app
namespace: default
spec:
serviceAccountName: default-ns-sa
fetch:
# I want to append "subPath: config-step-2-template" to the end of the git option.
- git:
url: https://github.com/vmware-tanzu/carvel-simple-app-on-kubernetes
ref: origin/develop
template:
- ytt: {}
deploy:
- kapp: {}
```

This is currently achieved with the following:
```
#@overlay/match by=lambda _,l,r: "git" in l
```

Full example in the ytt playground:
https://carvel.dev/ytt/#gist:https://gist.github.com/voor/02c1d2e2d3c08fe42fd2df7659bc386a

**Describe the solution you'd like**
I think this is a use case that might come up often enough to justify looking at either adding this to a FAQ or creating a corresponding matcher.

**Anything else you would like to add:**
I never would have figured this out without phoning a friend.

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start with the linked ytt playground example and compare the existing overlay/match lambda behavior with the requested key-existence check. Decide whether the completed issue is a documented FAQ entry or a corresponding matcher, and verify the example works with the chosen approach.

Written by the indexing model from the issue text.