Implement class-based API for graph declaration

[skovbasa]

Implement and process class-based graph components. Old and new apis have to coexist to enable gradual migration.

• field as a function returning field context.
• field_link as a function returning link context.
• Node through either a base class or a class decorator.
  • Add default resolvers for field and field_link.
  • Transform field and link names to camelCase automatically. Support name overriding.
  • Add types from annotations to fields and links.
    • Process default scalar types (int, float, str, bool, optional, list).
    • Process Node types. Maybe add link[Node] for a future.
    • Process custom scalar types.
    • Process interfaces.
    • Process unions.
    • Process enums.
  • Transform class-based Node to a regular one at graph initialization.
• Interface through either a base class or a class decorator.
  • No support for resolvers.
  • Transform class-based Interface to a regular one at graph initialization.
• Union as typing.Annotated[Node1 | Node2 | Node3, hiku.union('UnionName')].
  • Transform class-based Union to a regular one at graph initialization.
• Directive unchanged.
• Option unchanged.
• Enum unchanged.
• Lazy resolve Node types with typing.Annotated['Node1', hiku.lazy('.some_node')].