Add ItemAvailability value "DropShipped" (#4483)

[frankforte]

Add the value "DropShipped" for ItemAvailability, to signify products shipped by a third party.

[MatthiasWiesmann]

Please do not edit the core schema file, create new files in https://github.com/schemaorg/schemaorg/tree/main/data/ext/pending with a number matching the relevant bug.

[fthobe]

@frankforte there was a discussion about that here in #2651

There's some point in having a distinction between dropshipping stock and regualar stock, but keep in mind that this difference can also be solved by definition of delivery windows other added information you can provide around a product (e.g. a well defined shipping method).