Provider type exported by this package should be compatible with provider type exported by `eth-json-rpc-middleware`

[mcmire]

It's very common to see this in our code:

import { PollingBlockTracker, Provider } from 'eth-block-tracker';
import { JsonRpcEngine } from 'json-rpc-engine';
import { providerFromEngine } from '../src';

const engine = new JsonRpcEngine();
const provider = providerFromEngine(engine);

const blockTracker = new PollingBlockTracker({
  provider: provider as Provider,
});

We shouldn't have to typecast the provider like this.