Allow execution to be cancelled

[glasser]

Once execution begins, there's no way to stop it from running until completion.

However, servers may want to be able to stop execution. For example:

• Servers may have a response timeout; there's no use continuing to execute fields after the server has returned a "request timed out" error.
• Servers may want to avoid OOM attacks via nested lists by dynamically enforcing a maximum number of resolved fields and cancelling once too many fields have been resolved.

It would be helpful if execute (and experimentalExecuteIncrementally, etc) had a mechanism for cancelling execution. This could be based on web-style AbortSignal/AbortController or a different API style.