Expose sha256 function to rhai script

[Meemaw]

Is your feature request related to a problem? Please describe.
We are using a variant of persisted queries. Clients will send sha256 query signature within a request header, and the server validates that this matches the request body, and rejects the query otherwise.

With federation we cannot do that on the subgraphs anymore, because the query that is sent to subgraphs is modified so the signature obviously won't match.

Describe the solution you'd like
It's already possible to access request body, and headers within a rhai script, but the missing part is the sha256 function/module.

Describe alternatives you've considered
N/A

Additional context
I think exposing sha256 might be useful for other usecases as well.

[abernix]

Thanks for opening this Let's go ahead and track this in the existing #2025, which is essentially trying to capture the same thing. Please do add your use-case concretely for SHA256 (though that was definitely inclusive of my consideration when I opened it).

Duplicate of #2025