[feature] Bind way to provide (and create your own) handlerProvider

[zth]

handlerProvider is a way of letting you write custom handlers (think middlewares that run before a specific field is merged into the store, allowing you to change anything in the store as a field is added or updated in a centralized place). This is done by defining a handler and then referencing that on a field using the @__clientField(handle: String!) directive.

The magic @connection does is implemented internally by providing a handler for instance.

There should be bindings and docs for this.

[zth]

This isn't a feature that's advertised anything by Relay, so we'll drop this for now.