Link core extension statically

[simolus3]

Currently, we don't link the core extension at all on native platforms. Instead:

1. We ask users to manually depend on it and then load it as a path.
2. For our tests, we download desktop libraries and patch the link task of tests to include it.

Following this setup is annoying and error-prone for users, since they have to manage this dependency outside of Gradle and remember to update it every time they upgrade the Kotlin SDK.

This fixes the issue by linking the core extension statically: The :core package uses a cinterop definition with a downloaded static library attached to core extension releases. This embeds the core extension with the built klib, and users don't have to rely on external dependenies anymore. Additionally, it simplifies our loading setup: Previously, we had to use sqlite3_load_extension with a resolved framework path. With static linking, we can simply call sqlite3_auto_extension with the sqlite3_powersync_init function pointer to load the extension.

I've tested the demo app in a simulator and a local framework build for the Swift SDK (that currently causes additional test failures there because Kotlin doesn't support the old sync client anymore, but we're able to load the extension).

Closes #339.

[stevensJourney]

The changes here look good to me. This makes the setup steps much simpler for users. I can't think of any negative repercussions for these changes.