* Introduce generalised proxies to polkadot

* Introduce proxy to westend

* Add proxy to Kusama.

* Fix

* migrate subsystem expansion PR from w3f/research

As the file at this new location included changes not present in
https://github.com/w3f/research/pull/87, this is effectively a
rebase, applied manually. I believe that I have successfully retained
all of, and only, the intended changes.

* add section on collators

* note why the overseer is the terminal message point for the validation subsystem

* add detail about how the validitiy system gets PoV blocks

* rename to Validity Subsystems Overview

* get rid of changes to the implementor's guide

I think it makes most sense to track and review the subsystems
overview in a different PR than the one which adds content to
the implementor's guide.

* punt misbehavior arbitration details to the future

* empty commit to rerun CI

* Add block event forwarding into the Overseer

* Add a doc comment

* Fix a doc comment

* Adds start and stop work heartbeats.

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

* Fix code after suggested changes

* Finalizing stops work on earlier lower blocks.

* Fix func parameter and flaky test

Co-authored-by: Bastian Köcher <[email protected]>

* note pruning of orphan blocks in the overseer description

* remove note about finalization events being maybe not needed

* Make it work

* Upsub