Create webhook verification custom exceptions

[gcsuk]

Description

When calling VerifyHeader it would be good to know the difference between verification exceptions and any other possible exception. Currently, generic Exceptions are thrown, if we change this to custom exceptions they can be caught and handled specifically.

After this change we can do:

 try
 {
    ...
    webhookService.VerifyHeader(body, header, secret, 300);
    ....
 }
 catch (WorkOSWebhookException ex)
 {
     // Handle webhook exception
 }
catch (Exception ex)
{
    // Other exception handling
}

This is a non-breaking change as the custom exception inherits Exception, so any existing consumer handling these will still catch them.

Fixes #176

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.

[BoatPartyJesus]

This would actually be useful for us too for the additional context that comes with specific exceptions

[gcarvelli]

Looks great, thanks for the contribution!

[gcarvelli]

This has been released in WorkOS.net version 2.4.0.