Implement class-based API for graph declaration

[slavikovbasa]

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.
• [x] Directive unchanged.
• [x] Option unchanged.
• [x] Enum unchanged.
• [ ] Lazy resolve Node types with typing.Annotated['Node1', hiku.lazy('.some_node')].