* copy prop_breakdown tests into a separate file

* add a new endpoint for custom props

Duplicate existing goal prop breakdown as a building base for the
new endpoint

* stick to original metric names + CR definitions

We currently use CR with two different definitions, which is inconsistent
and should be changed. This commit just documents the difference for the
time being.

* basic prop breakdown without goal filter

* increase % metric precision to one decimal place

* add some tests without goal filter

* silence credo for TODO comment

* use events metric instead of pageviews

* review feedback

* inline add_cr_a instead