Support custom C++ types for GraphQL scalars via @cppType directive

[Copilot]

Custom scalars (e.g. BigInt) that can't be represented by response::Value were previously unsupported — schemagen always generated resolver and argument accessors typed as response::Value, giving no way to plug in custom storage, serialization, or deserialization.

Schema-level opt-in

• New schemagen-only directive @cppType(name: String! header: String), applied to a scalar declaration, maps it onto a custom C++ type instead of response::Value:

  scalar BigInt @cppType(name: "bigint::BigInt" header: "BigIntScalar.h")

• name is used wherever the scalar appears in generated resolver/argument signatures; header is added as an #include in the generated schema header.
• Scalars without @cppType continue to generate identical code (response::Value), so this is fully backward compatible.

Generator changes

• SchemaLoader: parses @cppType and resolves the associated C++ type/header for a given scalar.
• SchemaGenerator: emits the custom-scalar header include and substitutes the custom type into argument/result accessor signatures.

Runtime changes

• GraphQLService.h: added a CustomScalarArgument<T> trait — specializing it as std::true_type tells the runtime to treat T as a scalar argument (nullable → std::optional<T>) rather than a generated input object (std::unique_ptr<T>).
• Users bridge the custom type to/from the wire format by specializing service::Argument<T>::convert, service::Result<T>::convert, and service::Result<T>::validateScalar — the runtime's AwaitableScalar<T>/ModifiedResult<T>/ModifiedArgument<T> were already generic over T, so no changes were needed there.

Example

Added samples/scalar, demonstrating a BigInt scalar backed by std::int64_t (which doesn't fit in the 32-bit response::IntType), serialized on the wire as a JSON string, plus corresponding tests in test/ScalarTests.cpp.

Docs updated in doc/scalars.md and doc/directives.md to describe the new directive and conversion hooks.

• Fixes Unable to create BigInt scalar #346