feat: expose engine FFI API to a Dart consumer (prototype 3)

[bassosimone]

The general idea here is to avoid returning -1, having a more uniform API, and allowing for sync calls.

It's a bit more work on both sides but it's more regular, so maybe this change is for the greater good?

This will be a third pull request because it varies from https://github.com/ooni/probe-cli/pull/850 mostly in the way in which we interface at the FFI level.

Reference issue: https://github.com/ooni/probe/issues/2197