Added WinAppSDK background task sample

[godlytalias]

Description

This sample application demonstrates how a WinUI3 app can register Background Task. In this sample usage of ToastNotificationHistoryChangedTrigger is demonstrated which can be easily changed to any supported triggers. On clicking the button on the UI, background task gets registered and a toast notification will get pushed, when user deletes this toast notification background task gets triggered and push another toast notification to notify that BackgroundTask have been triggered.

Target Release

1.5

Checklist

• [x] Samples build and run using the Visual Studio versions listed in the Windows development docs.
• [x] Samples build and run on all supported platforms (x64, x86, ARM64) and configurations (Debug, Release).
• [x] Samples set the minimum supported OS version to Windows 10 version 1809.
• [x] Samples build clean with no warnings or errors.
• [x] [For new samples]: Samples have completed the sample guidelines checklist and follow standardization/naming guidelines.
• [x] If I am onboarding a new feature, then I must have correctly setup a new CI pipeline for my feature with the correct triggers and path filters laid out in the "Onboarding Samples CI Pipeline for new feature" section in samples-guidelines.md.
• [x] I have commented on my PR /azp run SamplesCI-<FeatureName> to have the CI build run on my branch for each of my FeatureName my PR is modifying. This must be done on the latest commit on the PR before merging to ensure the build is up to date and accurate. Warning: the PR will not block automatically if this is not run due to '/azp run' limitation on triggering more than 10 pipelines.

[godlytalias]

/azp run SamplesCI-BackgroundTask

[azure-pipelines[bot]]

Commenter does not have sufficient privileges for PR 366 in repo microsoft/WindowsAppSDK-Samples

[azure-pipelines[bot]]

Commenter does not have sufficient privileges for PR 366 in repo microsoft/WindowsAppSDK-Samples

[godlytalias]

Sample as described in #333

[Saharsh979]

/azp run SamplesCI-BackgroundTask

[azure-pipelines[bot]]

No pipelines are associated with this pull request.

[godlytalias]

Samples added in #392