Add an event dispatcher implementation to the update or system module

Problem/Motivation

I would like to be able to run processes, either before or after database updates, and at the moment this is not as easy as it might be.

Steps to reproduce

Proposed resolution

Remaining tasks

Implementation
Community feedback

User interface changes

N/A

API changes

I propose adding an event dispatcher to the update module to allow hooking into this process via an event subscriber. Thoughts?
Possible events added could include:

DB_UPDATES_STARTED
DB_UPDATES_FINISHED

Personally, I'd be happy with just the DB_UPDATES_FINISHED event.

Data model changes

N/A

Release notes snippet

New Events added to the update module
DB_UPDATES_STARTED emmitted immediately before updates start
DB_UPDATES_FINISHED emmitted immediately after updates finish