Allow tasks to return products.

[tobiasraabe]

Changes

• Allow tasks to produce by returning values. The produced nodes are specified as return-type annotations.

  def task_example() -> Annotated[str, PathNode("out.txt")]:
      return "Hello, World!"

• Allow a similar interface via @pytask.mark.task(produces=...) for the Python API to declare product types for return values.

Todo

• Rewrite all existing node types such that they use .load and .save

[codecov]

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