USD_UserExportedAttributesJson write custom Data

Currently when using the USD_UserExportedAttributesJson attribute to tag custom attributes in an object to export, only already recognized attributes by the plugin like usdAttrName or usdAttrType are used, the rest of the keys are discarded.

It could be useful to have the rest of non recognized attributes as customData of the usd attribute, for example:

USD_UserExportedAttributesJson in a pPlane1 mesh writing my_attibute_one of type int:
`{
"my_attribute_one" : {
"usdAttrName" : "attrOne",
"affectsSubasset" : True
}
}`

Would result in:

`
def Mesh "pPlane1"
{
custom int attrOne = 0
customData = {
bool affectsSubasset = True
}
`

Start at the USD_UserExportedAttributesJson export entry point and trace how recognized keys such as usdAttrName and usdAttrType are handled. Check how the remaining keys should become the USD attribute's customData, using the pPlane1 example as the expected result; done means affectsSubasset and similar unrecognized values are preserved.

Written by the indexing model from the issue text.