API/ENH: Allow Interval to be a first-class type via an IntervalBlock internal type

[jschendel]

This has been mentioned in passing in a few other issues, so making an official issue to document this. In the same spirit as PeriodBlock (#7964).

I've started working on an interval accessor, so it seems likely that there will be more exposure to Interval objects in a Series/DataFrame, hence this becoming more relevant.

xref #19272

[TomAugspurger]

FYI I have a branch doing this on top of the extension array stuff. Will make a PR in a couple days.