Add multi-value Categorical data Type

Currently he have two possible export data types from Document properties:

• img : used for blob properties with images
• txt : used for String fields, dictionary field, and blobs with extracted text.

We should add a new data type : cat (Category). The new data type is for properties of the kind Dictionary and should be multi-value.  The txt data type would not be used any longer for Dictionary properties. 

tfRecord

The actual export format should not change. a category is stored as text (byte stream), but with multiple dimensions, not one as for text. 

AI_Corpus

While creating the AI_Corpus Document in the cloud service, we should allow for cat fields. Cat is already defined in the  FieldDataType Vocabulary.

Update the platform's AI_Corpus document definition to allow also a Category data type.