feat(audit): fix broken instance-related events

Summary

fix(audit): fix broken instance-related events

This patch reintroduces instance-related events (e.g. join_instance) which were accidentally disabled in a bugfix of the DML handler (see commit 52ed9237 for more information) due to the lack of tests.

Furthermore, this patch adds the following events:

• expel_instance
• change_config, which tracks changes in _pico_property

This was the original intended change of the patch, but the apparent breakage of the preexisting events called for action.

Part of #349 (closed)

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if necessary) A follow-up doc issue is created in picodata/docs and linked here