Added a new custom_properties_namespace USD export option, to
allow replacing or omitting the current default "userProperties"
namespace prefix.
Note that this option does not apply to names that already have a prefix
(e.g., it would apply to name "bar" but not "foo:bar"). It also does not apply
to the internal Blender "object_name" and "data_name" properties which
always have the prefix "userProperties:blender".
Also added logic to handle ":" namespace delimiters in property names.
Pull Request: https://projects.blender.org/blender/blender/pulls/124067
94c184d2a7