Add extensions support to GraphQLRequest

[magicmark]

Summary

Adds an optional extensions parameter to GraphQLRequest, as defined in the GraphQL over HTTP spec. When set, it is included as a top-level key in the request payload alongside query, variables, and operationName.

No transport changes needed — all transports already serialize request.payload.

request = GraphQLRequest(
    "query { viewer { name } }",
    extensions={"persistedQuery": {"version": 1, "sha256Hash": "abc123"}},
)
result = await session.execute(request)

Closes #590

Test plan

• Unit tests for extensions in payload, copy, override, and serialize_variable_values
• Integration tests for execute, subscribe, and batch across aiohttp, httpx, requests, and websockets

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fb869a) to head (562a4c2).
⚠️ Report is 59 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #591    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3321   +413     
==========================================
+ Hits          2908      3321   +413     

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[magicmark]

@leszekhanusz what do you think about this approach vs a new argument to session.execute (and friends), or even extra_args?

(extra_args seems to be something intended to be passed directly to the transport though, so that's probably not a good fit)

[leszekhanusz]

Looks good to me! Much better than having to add an extensions attribute to ALL the execute and subscribe methods.

[leszekhanusz]

Thanks!