This adds a toggle to node group inputs exposed in the modifier to use
an attribute instead of a single value. When the toggle is pressed, the
button switches to a text button to choose an attribute name. Attribute
search isn't implemented here yet.

One confusing thing is that some values can't be driven by attributes
at all, like the size of a primitive node. In that case, we should have
a node warning, but that will be separate since it's more general.
We can also have an option to turn off this toggle in node group
input settings.

The two new properties for each input are stored with the same name
as the value, but with "_use_attribute" and "_attribute_name"`
suffixes. The properties are not added for socket types that don't
support attribute input, like object sockets.