Let users provide custom `executeFn`

[XiNiHa]

Is your feature request related to a problem? Please describe
In the recent work on graphql-js for bringing @defer/stream, they introduced a new interface for executing with @defer/stream, which is called experimentalExecuteIncrementally. However, currently there's no way to tell Envelop to use this execution function instead of execute.

Describe the solution you'd like

Add an option to let users provide their own executeFn.

Describe alternatives you've considered

Provide an option to enable incremental delivery. I've put this as an alternative only because the incremental execution function has experimental at the front of its name, which makes it unreliable to depend on the interface from the library's end.

[saihaj]

Hey! We are working on next major release #1487 which would allow you to pass in custom functions

[ardatan]

But I think it is possible currently with onExecute hook.
https://github.com/n1ru4l/envelop/blob/cea2df214e4d598f1206b1e066bf285960011378/packages/types/src/hooks.ts#L329

{
   onExecute({ setExecuteFn }) {
       setExecuteFn(myExecute);
   }
}

[XiNiHa]

@saihaj lovely 🥰
@ardatan Oh right, just realized that this reassigns executeFn with custom implementation provided, missed that!