Extend web push support

[MarkusEngelHahnSoftware]

Hi, this adds some missing annotations for the recently added BrowserRegistrationDescriptions and registers them with the serializer. Besides that, I added some convenience methods to NotificationHubClient in accordance with the already existing ones.

Things to consider before you submit the PR:

• [X] Are tests passing locally?
• [X] Are the files formatted correctly?
• [ ] Did you add unit tests?
• [X] Did you test your change with either the sample apps that are included in the repository or with a blank app that uses your change?

Related PRs or issues

314

Misc

"CreateBrowserTemplateRegistrationAsync" methods are currently not added it INotificationHubClient.

[MarkusEngelHahnSoftware]

@microsoft-github-policy-service agree company="Hahn Softwareentwicklung GmbH"

[timvandesteeg]

Can someone please review this PR?

[lomagdal2]

lgtm