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

[rablador]

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.

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This change is 

[buggmagnet]

Reviewed 24 of 32 files at r1, 6 of 7 files at r2.
Reviewable status: 29 of 32 files reviewed, 6 unresolved discussions

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ios/build-rust-library.sh line 57 at r2 (raw file):

      if [ $IS_SIMULATOR -eq 0 ]; then
        # Hardware iOS targets
        rustup target add aarch64-apple-ios

nit
As discussed with the team, this was only a temporary fix for devs, we should remove there 2 new lines.

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ios/MullvadRustRuntime/MullvadApiCompletion.swift line 16 at r2 (raw file):

    let completionBridge = Unmanaged<MullvadApiCompletion>
        .fromOpaque(completionCookie)
        .takeUnretainedValue()

this should be takeRetainedValue() otherwise we are leaking memory with every request.

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ios/MullvadVPN/AddressCacheTracker/AddressCacheTracker.swift line 122 at r2 (raw file):

        return Date()
//        return _nextScheduleDate()

Reinstate the commented line

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ios/MullvadTypes/AsyncExample.swift line 0 at r2 (raw file):
Delete this ?

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ios/MullvadREST/ApiHandlers/RESTRustNetworkOperation.swift line 149 at r2 (raw file):

extension MullvadApiResponse {
    public func restError() throws -> REST.Error? {

This should either throw or return an Optional value, not both.

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ios/MullvadRustRuntime/MullvadApiContext.swift line 18 at r2 (raw file):

        if context._0 == nil {
            throw NSError(domain: "", code: 0)

nit
We should have a custom error code that can identify why the request failed maybe ?
This way, debugging potential errors will be less of a hassle