Add initial api for SDK

[i1i1]

This pr adds mock API for SDK, which exposes same functionality as this typescript example. Overall, it follows those rules:

• Everything should be extendable
  • all the enums and public structures (which expose fields) are not exhaustive
  • most of the structures implement default
  • structures are constructed using builder pattern (look at the Node and Farmer)
  • most methods/functions/callbacks are async, so that if we don't need async functionality now, we could use that in future
• Stateless
  • There is no reading/writing to a config file, as this kind of state management should be done at the different level (maybe subspace cli)

The simplest example of running both node and farmer would look like this:

let mut node = Noder::builder().build(Directory::Tmp).await.unwrap();
node.sync().await;

let mut farmer = Farmer::builder()
    .build(
        PublicKey::from(0; 32]),
        node,
        PlotDescription::with_tempdir(ByteSize::gb(10)),
    )
    .await
    .unwrap();
farmer.sync().await;
farmer.start_farming().await;
loop {}

[nazar-pc]

I don't think I'm a good person to review this, I'm 100% out of context on what this is supposed to do and what the goals are.

[ozgunozerk]

LGTM

[achiurizo]

Approving to unblock this. Let's make incremental fixes as reviews come in so that we can start moving forward on SDK 🙏