(app): Add on_load_state_dict and on_save_state_dict

[tchaton]

What does this PR do?

This PR introduces an API to reload dynamic component trees.

The current strategy is the following. We collect all the states. On reload, if a component state doesn't have its component, we give the responsibility to its closest parent to resolve it and then the cascade of instantiation is done.

class FlowReload(LightningFlow):

    def run(self):
        if not getattr(self, "w", None):
            self.w = WorkReload()
        self.w.run()

    def on_load_state_dict(self, children_states, extras) -> None:
        self.w = WorkReload()

Fixes #<issue_number>

Does your PR introduce any breaking changes? If yes, please list them.

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[Borda]

@tchaton, mind checking the remaining comments?