Add library loading capacity to `_allow_dynamic_callbacks`.

[T4rk1n]

When _allow_dynamic_callbacks=True, callback can now import a component library and automatically loaded on the callback returns. Same restrictions as adding callbacks during callbacks, only guaranteed to work for a single user, meant to be used for development purposes.

[ndrezn]

@chriddyp / @hatched I think we should make a build of Dash based on this version and test whether we resolve the issue in App Studio with third-party component libraries. What do you guys think? Happy to create that build if so.

[ndrezn]

@T4rk1n: From @robertclaus, this change was tested in App Studio and works as expected 🥳 . See: https://github.com/plotly/notebook-to-app/issues/401#issuecomment-2269443734