Add OSGi manifest headers for jars

[bdhoine]

The gradle plugin that is added to the build is provided by BND tools: https://github.com/bndtools/bnd/tree/master/gradle-plugins. General info about BND tools can also be found on https://bndtools.org/

Adobe Experience Manager is also using the Maven variant for example in their archetype to achieve OSGi compatible JAR's bundles: https://github.com/adobe/aem-project-archetype/blob/master/src/main/archetype/pom.xml#L214

resolves #86

[jack-berg]

There's a similar PR in opentelemetry-java right now: open-telemetry/opentelemetry-java#6669

My comment there is applicable here as well:

As someone without OSGi experience, I think its important to add some sort of end-to-end integration test. A sample app built with OSGi which consumes these artifacts and demonstrates it works as intended.

[bdhoine]

@jack-berg yes I noticed, @royteeuwen and myself are working on some details how to solve the current issues we are seeing while testing, this topic definitely needs some more work!

in the meanwhile I'm marking this one as draft