(app): Add load_state_dict and 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.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

cc @Borda

[lantiga]

A few comments on capturing a "resumable state" from Work.

[Borda]

@tchaton, mind checking the remaining comments?