Add support for TDS_SSVARIANT structure in tds.py

[n3rada]

Hello @gabrielg5 👋

This pull request adds support for parsing SQL Server sql_variant types in the TDS protocol implementation. The main change is the introduction of the TDS_SSVARIANT class, which can parse and extract the value of a sql_variant field according to the [MS-TDS] specification: https://learn.microsoft.com/fr-fr/openspecs/windows_protocols/ms-tds/2435e85d-9e61-492c-acb2-627ffccb5b92

I is useful for queries such as:

SELECT value_in_use FROM sys.configurations WHERE name = 'show advanced options';

Of course, for this specific case, it could be "bypassed" using CAST(value_in_use AS INT), but i thought it could be great to add this type inside the code directly.

Before:

After:

Best regards