Move channel tags for the import texture set operator to an editable JSON file.

[LoganFairbairn]

No description provided.

[LoganFairbairn]

Some teams use custom naming conventions for texture file types that aren't covered in the default material channel tags for the import texture set operator. To support these custom naming conventions properly, it should be possible to export / import the names in json format so it can be shared between team members.