Add generic type safety hints

[larohra]

The package is lacking type safety in a bunch of places which are causing partially unknown type safety warnings -

Type of "schedule_new_orchestration" is partially unknown
  Type of "schedule_new_orchestration" is "(orchestrator: ((OrchestrationContext, TInput@schedule_new_orchestration) -> (Generator[Task[Unknown], Any, TOutput@schedule_new_orchestration] | TOutput@schedule_new_orchestration)) | str, *, input: TInput@schedule_new_orchestration | None = None, instance_id: str | None = None, start_at: datetime | None = None, reuse_id_policy: OrchestrationIdReusePolicy | None = None, tags: dict[str, str] | None = None, version: str | None = None) -> str"

Though they're not affecting runtime stability, its good to have clean type hints to ensure Pylance can always infer hints cleanly in any environment.

Sample PR with some changes - #91
It would be great to see generic type-hints for all functions.

[larohra]

Additional places where type safety is missing -

class OrchestrationContext(ABC):

  @abstractmethod
      def wait_for_external_event(self, name: str) -> Task:

  @abstractmethod
      def create_timer(self, fire_at: Union[datetime, timedelta]) -> Task:

[andystaples]

Didn't see these additional places until after #104 was merged, keeping this issue around until those can be fixed too