Support Standard Serialization Formats

[sidjha1]

Is your feature request related to a problem? Please describe.
Currently the df2text function uses some custom serialization format. It would be good to support standard formats like JSON, XML, etc. since it better fits model training distributions. In fact, Anthropic recommends you use XML.

Describe the solution you'd like
There should be a serialization_format setting that is read inside the df2text function to do the proper formatting. The nice thing about pandas is that it already gives you things like to_xml and other functions to help with serialization.