Use 'pluggy' to manage extension points

[ahal]

Currently we use an adhoc mish-mash of "register" functions and monkeypatching in order to extend Taskgraph. These places are not well documented and tend to be implemented inconsistently.

Pluggy is the extension manager used by Pytest and several other prominent projects. It would allow Taskgraph to expose well defined hook points that "plugins" can then call.

This would give extensions (like mozilla_taskgraph or project specific Taskgraph modules) well defined places to insert things into Taskgraph. We could then get rid of all the hacky monkeypatching and random importing of modules to trigger decorators.