Add a "boot" phase

[DanielCoulbourne]

Adds a mount phase which happens before auth and validate.

This phase can be used to set up event props, etc.

Useful if you want to register custom attributes that modify properties like so:

class SomeEvent extends Event
{
    #[NowIfNull] // We can register a listener on the mount hook that sets this to now() if it is not passed.
    public ?Carbon $when_did_the_thing_happen = null;
}

[inxilpro]

Can we call it boot or construct instead of mount?

[joshhanley]

Would it also run when an event is replayed? If so, then I think boot() would be better

[inxilpro]

Reviewed with Daniel. We need to address the differences between this and #145 but otherwise looks good.