Fix IWebhook.GetUdi() and IEntity.GetUdi() extension methods

[ronaldbarendse]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Although PR #15267 includes most of the boilerplate code to make Webhook implement IEntity, the GetUdi() extension method wasn't updated to use the interface and the IEntity.GetUdi() extension method didn't support the webhook entity type. This PR fixes both of these issues in the first commit.

I've also taken the opportunity to refactor the code in the second commit, so it's much more compact and readable by using:

• ArgumentNullException.ThrowIfNull() for all null checks (although we're already in a nullable context);
• switch expressions, which also takes care of detecting unreachable code, in case multiple types would match (e.g. IContent and IContentBase).

Finally, I've also added a quick unit test to verify the GetUdi() extension methods do what they should 😄

[AndyButland]

Looks good to me. I just followed the "be consistent trail" a little bit more and added a "webhook builder" to use in the added unit test.

[Zeegaan]

Lgtm 👍