Expose ExecutionResult extensions

Currently there is no way for the middleware or resolver to set data in the `extensions` field of the `ExecutionResult` response object. This prevents the use of tools like performance tracing or metadata injection.

Start by locating ExecutionResult and the middleware or resolver execution paths in the Python GraphQL implementation. Determine how extensions should be supplied to the response, then add coverage showing that middleware or a resolver can populate ExecutionResult.extensions.

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