Strongly typed subscriptions

[elderapo]

It would be cool if subscriptions were strongly typed. Right now they look like this:

const stream = await client.subscribeTransactions({
  includeMempoolAcceptance: true
});

stream.on("data", tx => { // type of tx is any
  tx.??????
});

Maybe we could switch API a little bit like:

const cancel = client.subscribeTransactions({ ...opts }, (tx) => { //  type of tx wil be TransactionNotification

});

cancel(); // to stop listening

[jcramer]

Another great suggestion, we can do that.