Make is possible to delete metadata from a thing

[thjaeckle]

With #680 it is possible to put/set arbitrary metadata to a digital twin (thing).
It however is not possible to delete a once set metadata - it is only possible to e.g. set the metadata's value to null.

In scenarios where deletion is favorable it would be good to make deletion possible.

The syntax of deletion could look like, as an additional header "delete-metadata":

delete-metadata:
[
  "issuedAt"
]

[jufickel-b]

As the header names for metadata now contain verbs, this is a nice approach IMHO.