Allow execution to be cancelled #3764
Description
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.