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:

• Add 'extensions' to request graphql/graphql-spec#976

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

Solution

• 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