pdfXFA: Add support for rich text values in form state

In previous versions of pdfXFA, form flattening lacked support for rich text values. This resulted in the loss of rich text content when flattening XFA form fields. 

However, in pdfXFA version 3.0.1 rich text data in form fields is now supported. 

The following example will replicate the issue in previous versions, as well as demonstrating the fix in the current version. 

##GITHUB:https://github.com/pkjuak/Jdoodle/blob/main/RichTextXFATest##

##GITHUB:https://github.com/pkjuak/Jdoodle.NET/blob/main/RichTextXFATest##

Resources:

richTextFieldNoDataBinding.pdf
result.pdf