More core adaptations

This takes on @shykes last commit to continue extensions management and `Context` providing to loaded extensions. I tried my best to push things forwards while building on the changes introduced by d8e536ad1d4a6, but tbh I don't understand them all so I hope the result makes sense.

The good news: it builds. The bad news: it crashes at daemon initialization as more and more code is now relying on `state.Scope()` (which is not yet implemented in `GitState`).

What still seems to be missing for a running daemon:
* Implementation of `extensions.state.GitState.Scope()`
* Implementation of `extension.simplebridge.Controller` which registers the driver
* Implementation of `sandbox` to give the driver somewhere to put its shiny network interfaces