Add List Extensions methods to manage Webhooks subscriptions

[ypcode]

Q	A
Bug fix?	no
New feature?	yes
New sample?	no

What's in this Pull Request?

List Extension methods to manage List Webhooks subscriptions and all the underlying utilities + related unit tests

[msftclas]

@ypcode,
Thanks for your contribution.
To ensure that the project team has proper rights to use your work, please complete the Contribution License Agreement at https://cla.microsoft.com.

It will cover your contributions to all Microsoft-managed open source projects.
Thanks,
Microsoft Pull Request Bot

[VesaJuvonen]

Hi Yannick,
please remember to sign the CLA at https://cla.microsoft.com. Submission looks good with quick check, but we'll have a closer look on upcoming days. Thx.

[ypcode]

Hi @VesaJuvonen, I actually did sign it, and I have a copy the signed document, I was expecting this status to change automatically, I'll redo it ;)

[jansenbe]

Thanks @ypcode, you did a great job on the PR. I've merged your PR but also did some additions and refactoring.

[ypcode]

You're welcome ;) It's a great honnor to bring my modest contribution to this project ! I plan to do a lot more.

By the way, Can somebody tell me why the CLA is still pending ? I sent an email to the support address but no response yet...