introduces the NotificationContext and ContextBuilder classes.
the only specialization currently is for the PR comment.

the context builders generate the context details necessary to create and
send the message later on. Each step is called in order, and if one step fails
we bail with an exception. This means the notification can't be sent.

Create a descriptor for notification context fields so that we fail with a
custom exception if that field was not loaded but we are trying to access it.

With the added benefit of reducing the amount of code a little bit :E

I added typehints to the fields (e.g. `pull: EnrichedPull = NotificationContextField[EnrichedPull]()`) because my VSCode langauge server was not being helpful with the generics... but leaving the generics in any case.

This lets us only load the necessary info once and re-use the calculations for subsequent contexts that need to be built.

* improve docstrings
* change bundle analysis PR comment context name