Cast to unlimited varchars in MS SQL (especially UUDS)

[nolar]

Otherwise, it fails with something like this in tests:

    sql_code = 'SELECT min(CONVERT(varchar, [col])), max(CONVERT(varchar, [col])) FROM [src_mssql_uniqueidentifier_postgresql_char100_50_rrp16]'
data_diff.databases.base.QueryError: ('42000', '[42000] [Microsoft][ODBC Driver 18 for SQL Server][SQL Server]Insufficient result space to convert uniqueidentifier value to char. (8170) (SQLExecDirectW)')

That's because it cannot convert a 36-symbol UUID to 30-symbol varchar. MS SQL Server uses the default length of 30 if other length is not specified.