Support `extensions` in `GraphQLRequest` (#1347, graphql/graphql-spec#976)

[tyranron]

Part of #1347

Synopsis

GraphQL September 2025 spec standardizes optional extensions field for implementation-specific details:

• graphql/graphql-spec#976

Juniper should be able to parse such implementation-specific details and provide them in juniper::Context during execution.

Solution

• [x] Add pub extensions: Ext field to GraphQLRequest, where Ext = Variables<S> by default. This way, user code could define any structured extensions as it desires, while by default the code will just work.
  • [ ] in juniper_actix
  • [ ] in juniper_axum
  • [ ] in juniper_hyper
  • [ ] in juniper_rocket
  • [ ] in juniper_warp
• [ ] Wire these extensions to Context in GraphQLRequest::execute() method. (TODO: describe how)

Checklist

• [ ] Tests are added
• [ ] Documentation is added (including Book)
• [ ] CHANGELOG is filled