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

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.

Start with the jextract-swift tool and the SwiftPM, Gradle, and Maven integration options named in the issue. First clarify how Swift and any generated Swift glue will be built, how Java wrappers will be generated, and how the result will connect to a Java build. Done means a defined, usable workflow for producing the wrapped library, whether through a standalone JAR or build-tool integration.