feat(kotlin): add logic & metadata for publishing

[lewisfm]

This PR contains the changes required to publish the Kotlin bindings to Maven.

• Adds the POM metadata needed for publishing to Maven Central
• Sets up Dokka for generating API documentation (docs are required by Maven Central)
• Sets up code signing via GPG (required by Maven Central)

To actually publish:

1. Make a Maven Central account https://central.sonatype.org/register/central-portal/
2. Set up GPG https://central.sonatype.org/publish/requirements/gpg/
3. Run ./gradlew publish -Psigning.keyId=GPGKEYID to create the jar package.
4. Upload kotlin/lib/build/distributions/deploy.jar to Maven Central https://central.sonatype.com/publishing
5. Wait for it to verify and then confirm it

Here's the package I made while testing: https://central.sonatype.com/artifact/io.github.doinkythederp/iroh

Note: For the Kotlin module to be multi-platform, it will need to contain the iroh cdylib built for multiple platforms.

Make your native library available on your classpath, under the path {OS}-{ARCH}/{LIBRARY}, where {OS}-{ARCH} is JNA's canonical prefix for native libraries (e.g. win32-x86, linux-amd64, or darwin). If the resource is within a jar file it will be automatically extracted when loaded.
https://github.com/java-native-access/jna/blob/master/www/GettingStarted.md

resources/
	darwin/
		libiroh_ffi.dylib
	linux-amd64/
		libiroh_ffi.so
	...etc...

[dignifiedquire]

thanks a lot! I will try and look at this this week

[lewisfm]

I imagine the best way to achieve a multi-platform JAR is to update the CI matrix to generate the dylibs as artifacts, then have one final JVM build step which pulls in all of them. Alternatively, it might be possible to use cross, but it seems to be non-trivial to cross-compile for macOS.