Implement an FFI to fetch API IP addresses using mullvad-api

We should test out if using the mullvad-api crate is feasible. An FFI that allows for calling out to https://api.mullvad.net/app/documentation/#/paths/~1v1~1api-addrs/get

We should then see if this works when:

• Connected over 4g
• Connected over WiFi
• Connected to a VPN when IAN is set up
• Blocked state when IAN is set up
• Proxies?

Code can be throwaway for now, but if we can make it production ready, we should.

The Swift interface should look similar to the one below:

protocol AddressFetching {
  func fetchApiAddresses() async throws -> [MullvadEndpoint]
}

Said interface would probably have to be implemented by passing a continuation to Rust for when the result is fetched.

/// Swift side for the continuation
struct FFIContinuation {
  let callback: UnsafeMutable<...>
}

@convention(c) func fetch_api_addresses_result(continuation: UnsafePointer<FFIContinuation>)

This task should be owned by an iOS team member, but they should feel free to reach out to any of the Rust developers for help and to pair-program on this task.

As part of this, we should also implement unit tests at least for the Rust parts.

To test that it works, lower updateInterval in AddressCacheTracker to a few seconds to trigger the update of address list. The log should get entries saying Address list: success([45.83.223.196:443]).