Support 'unknown' sdtype

[amontanez24]

Problem Description

The metadata updates in #1515 will add a new sdtype called unknown. This sdtype is when we were unable to pick an sdtype and a generic faker should be used.

Expected behavior

• When assigning transformers, if the sdtype is unknown, assign the following transformer

AnonymizedFaker(
    provider_name=None,
    function_name='bothify',
    text='sdv-pii-?????',
    letters='0123456789abcdefghijklmnopqrstuvwxyz'
)

Additional context

• The logic for assigning transformers can be found here. We will no longer need the case where we default to categorical transformers if all else fails since every column should have an sdtype or unknown.