feat: impl PubsubClient for MockProvider

[dereksione]

Motivation

The current implementation of mock provider doesn't support any of the eth JSON RPC endpoints that establish subscriptions. Supporting this would allow for unit tests and smoke tests for implementations that depend on these features.

Solution

This implementation extends the idea of the mock provider being a sink of pre-rendered responses piped in before beginning the testing process. There is a simple public facing function setup_subscription that takes all the previously pushed responses and converts them into a stream that can be subscribed to using any of the eth JSONRPC requests that create such subscriptions.

Internally, the implementation is a little convoluted because the PubsubClient takes an immutable reference to self. The requirements of the task is that we need the mutable sender of a stream that we can push things to. The solution implemented here uses an intermediary stream that is initialized before the subscribe function is called, and it forwards all of its data to the stream that is returned by the subscribe function in the PubsubClient trait.

PR Checklist

• Added Tests
• Added easy to use public facing API
• Add documentation

[DaniPopes]

Also needs a rebase and clippy fix.