feat: Add Run.on API to directly add an event callback

[ajbozarth]

Which issue(s) does this pull-request address?

Closes: #471

Description

This introduces the Run.on method that combines the Run.observe and Emitter.match calls into one more user friendly callback.

Included in this PR:

• The Run.on method and rework of Run internals to support it
• Updates to a few demos to leverage the new method
• Address mypy errors by moving the ContextVar storage to a global, as a ContextVar is intended to be

Checklist

• I have read the contributor guide
• I have signed off on my commit
• Tests are included
• Documentation is changed or added
• Commit messages and PR title follow conventional commits

[ajbozarth]

There can still be discussion around the new method's name. I went with Run.on but I also considered the following options if there's strong opinion to switch to one of those: observe_on, match, observe_match, on_event, or to try and overload observe itself (not trivial)