Subtree lifecycle hooks

[hansihe]

In order to support things like stateful components or to support phx-update/temporary assigns on top of managed data structures, we need the ability to support subtree lifecycle hooks.

This would enable us to run custom code for the following events:

• A subtree has been created
• A subtree has changed
• A subtree has been destroyed

For stateful components, the mapping would be as follows:

• Subtree created -> create the stateful component
• Subtree changed -> rerender the stateful component
• Subtree destroyed -> destroy the stateful component

Likewise, for phx-update/temporary assigns:

• Subtree created -> create the managed data structure
• Subtree changed -> perform append/prepend to the ClientList
• Subtree destroyed -> destroy the managed data structure