This is just a quick test to make custom attributes available in Cycles (e.g. attributes that have been created with geometry nodes).
Will continue to work on it soonish.
There are a couple of todos:
* Fix crash when attribute type changes, but the name stays the same (looks like there is a missing update in cycles).
* Potentially use different types for integer and bool attributes, currently they just become floats in cycles. This wastes some memory in many cases, but I'm not sure how Cycles handles other geometry types internally, or if everything should just be floats.
* Investigate potential conflicts with sculpt vertex colors (which are a subset of custom attributes).