Blender Git Loki

Cycles: support accessing custom mesh attributes

This makes custom mesh attributes available in Cycles. Typically,
these attributes are generated by Geometry Nodes, but they can also
be created with a Python script.

* The `subdivision` code path is not yet supported.
* This does not make vertex weights and some other builtin attributes
available in Cycles, even though they are accesible in Geometry Nodes.
All attributes generated in Geometry Nodes should be accessible though.
* In some cases memory consumption could be removed by not storing all
attributes in floats. E.g. booleans and integer attributes for which
all values are within a certain range, could be stored in less than
4 bytes per element.

Differential Revision: https://developer.blender.org/D10210