[eth-json-rpc-middleware] [fetch] Add support for cancelling requests

Currently a request may be retried up to 5 times. It should be possible to cancel a request outright without having to exhaust the maximum number of retries.

Similarly, it should be possible to cancel a request that hangs (`fetch` doesn't have a default timeout).

Locate the eth-json-rpc-middleware package and its fetch retry and request-handling entry points. Determine how cancellation should be exposed and verified; done means a request can be cancelled without exhausting retries and a hanging fetch can also be cancelled.

Written by the indexing model from the issue text.