Explore KMP as viable library

I'm quite confident we should be able to build a Kotlin Multiplatform using the bdk-android and bdk-swift libraries.

I'm starting to explore this a bit more and will share an example repo soon.

Start by reviewing the bdk-android and bdk-swift libraries and the Kotlin Multiplatform approach described in the issue. The issue says an example repository is forthcoming, so use that repo when available to evaluate whether the libraries can support a Kotlin Multiplatform build; done means documenting a viable path or its blockers.

Written by the indexing model from the issue text.