Build tooling to wrap up a Swift library for use in Java

[DougGregor]

The jextract-swift tool emits Java sources that wrap a Swift library for use in Java. We should make this use case as easy as possible, e.g., something that can integrate with a SwiftPM build (to build the Swift parts), create the corresponding Java wrappers, and integrate that into a Java build. Whether this is creating a standalone Jar file or integrating into Gradle/Maven is not yet clear.

Note that jextract-swift is very likely to need to generate some Swift glue code as well, which might itself need to be built with SwiftPM, so we'll need to account for that in the design.

[ktoso]

When we get to this, @simonjbeaumont has some prior experience on packaging up native libs in a jar so we can check in for some prior work he did on such.