jextract: extract Foundation.Date as Instant

[madsodgaard]

Adds support to JNI mode for extracting Foundation.Date as java.time.Instant.

[ktoso]

Neat :-) Please add info about it to the feature list docs

[ktoso]

because it's an unusual conversion I think we have to add actual docs about it

[ktoso]

I keep thinking about this from an overall project perspective, it is a bit random to bless a few types, but yes this one would work kinda well...

[madsodgaard]

@ktoso Hm, imo this project exists to provide a seamless Swift/Java interop story. Since we are generating Java code meant to be used on Java platforms, perhaps also by Java developers, I think it makes sense to bridge any matching types, such as UUID, Date,URL etc...

The alternative is that the developer has to use a generated type Swift.Date like we do for Data in FFM. That will probably often (for these kind of "utility" types") result in the developer having to convert their Java types to the Swift wrapped types. I understand why it makes sense for Data because of memory copying etc, but not sure that its the same motivation for the above types.