Add method for metadata keys to be unset back to their default values

[godind]

This is about manipulating metadata (baseDeltas.json) using PUT.

Currently, there is no mechanism to unset a key. This limits our ability to set metadata keys back to their default values once the key is present in baseDeltas.json.

For example:
We can, for a given path meta, use PUT to update displayName and:

1. Create/update the key with a given string value
2. We can set the value to an empty string ''
3. We can (not sure if supported yet) set value to null to indicate no value should be use

But we cannot remove the key, which would effectively unset the key and have said metadata key go back to its default value.

I think sk could have plugin that exposes metadata keys using a thread save jsonPatch mechanism, or do we already have something similar?

[godind]

Simplified and revised report.