refactor(core): Rework behaviors

[Ehesp]

This PR reworks how behaviors are handled internally, the public API doesn't change.

We now have 3 different behavior handler types:

• init: called on initalization of UI, and sets the initial state to loading. These behaviors are called once the auth state is ready from firebase. Once all resolved, the state is set back to idle.
• redirect: called on initalization after getRedirectResult has triggered - these could be used to handle upgrades and such.
• callable: requires manually calling from application code