[Sdk v3] Evaluate event systems

[bryanapellanes-okta]

To enable custom code to be injected into the request and response pipeline we intend to modify code generation templates to emit events. Rather than rolling our own event system it is preferable to integrate an existing package. The intent of this issue is to open the conversation up for anyone who wishes to express their opinions, preferences and concerns related to this topic. The list of packages currently under consideration are as follows:

blinker
PyDispatcher
pymitter
python-dispatch
pluggy
Events
zope.event
RxPy3
Louie
PyPubSub
pyeventdispatcher
buslane
axel
dispatcher
py-notify

[bryanapellanes-okta]

It may be worth considering implementing this as an injectable module that wraps the system chosen from the list of contenders. By providing a default reference implementation consumers could use the default or implement their own following the patterns we establish. Related to #408