Add new publishCIPackages target in Gradle to make it easier to control which targets to build on a given platform

[cmelchior]

With this change, it is now possible to define which targets to build using. It does this by introducing a new target publishCIPackages, which can be configured using a number of new Gradle properties:

realm.kotlin.mainHost=true
realm.kotlin.targets=<CommaSeperatedListOfPublications>
realm.kotlin.buildRealmCore=true
realm.kotlin.copyNativeJvmLibs=false

With this it is possible to get fine-grained control over exactly what gets built on a CI runner.

This PR is mostly moving things out of #881, so we can keep the review more focused.

[rorbech]

Logic seems ok, but implementation feels a bit cumbersome. I think I would have gone for a HostSpecifics-publication (#naming 🤪) and some conditional publishing that only published the host specific packages on publishHostSpecificPublicationTo<Repository>. That feels more aligned with the publishing infrastructure ... and would be really cool for local development if you could just do publicHostSpecificsToMavenLocal and install the artifacts needed on your platform.

But probably not worth redoing now 🤷