Support for Teradata's CLOB data type in row validations

Context: row validations
Source: Teradata
Target: BigQuery

Details:
Table in Teradata has a CLOB column which maps to a String column in BigQuery. When attempting row validations on this table, we are seeing the following error:

[Teradata Database] [Errpr 9881] Function 'TransUnicodeToUTF8' called with an invalid number or type of parameters

TransUnicodeToUTF8 cannot be applied to non-VARCHAR data types. Casting to VARCHAR normally happens for columns prior to this step, but this column is already a "string" field in ibis so the casting is not happening.

Adding support for the CLOB data type specifically would ensure the casting to VARCHAR occurs and this error is bypassed.