Support to update store from server when using partykit

[rbhalla]

Is your feature request related to a problem? Please describe.
Partykit supports event handling for events outside of client updates (e.g. alarms and disconnections). However, I can't see any easy way to update the store from the server side.

There is some ability to add an override for onRequest but it's not clear how to actually manipulate the store using this method.

Describe the solution you'd like
Ideally a tinybase like instance that allows you to manipulate the store like you would on the instance would be great.

Describe alternatives you've considered
I considered just faking messages/requests from the server as if they were client requests, but this seems brittle.