Add a context object in the onFetchRequest/onFetchResponse signatures

[rtarnaud]

I would like to be able to pair a RPC-JSON response with its request in the http transport. Currently, the only way to do it is to rely on extra query parameters added to the url but it is brittle solution as a node provider could decide to ban them.

Instead, we could rely on a context object that would be writable in onFetchRequest and readable in onFetchResponse:

onFetchRequest: (request: Request, context: object) => { context = <...>; // some local value }
onFetchResponse: (response: Response, context: object) => { // Use context to pair the response with the right request }

Please let me know if there's a more elegant way to do it.