Shared variables system

## 🪤 Context

Similar to the way synapse has entity locals, need a way to define and share variables between automation services. This will need to be driven by a UI to create and manage the default state of the variable.

The backend will work to actively preserve state for that shared variable, writing values to the sqlite database so they will persist across app reboots & random automation restarts

## 📬 Caveats

Also need a way to capture update events from these shared variables, similar to the way that entities/synapse entities are able to track state & emit updates.

Really want to use the shared variable as an update source for managed switches

## 🩻 Additional details