Add a MAUI (WinUI-only) client sample

[kevinchalet]

MAUI doesn't support the .NET Generic Host, so the new OpenIddict Windows integration cannot work natively without registering custom implementations of IHostEnvironment and IHostApplicationLifetime and using an IHostedService -> IMauiInitializeService adapter. This sample shows how to do that for MAUI on WinUI3.

Note: the custom implementation of IHostApplicationLifetime is very naïve as the host integration offered by MAUI is extremely limited. That said, it works, so... 😅

Hopefully a future MAUI version will improve things: dotnet/maui#2244.

[kevinchalet]

The build fails because installing the maui-tizen workload is required... to build a WinUI3-only MAUI app.
Why can't we have nice things, Microsoft?...

[kevinchalet]

Having to install the maui-tizen workload just for a sample is far from ideal so I won't merge this PR into the dev branch.

That said, if there's enough interest, I'll update the PR branch from time to time. If you're interested, let me know.

[kevinchalet]

Branch updated: https://github.com/kevinchalet/openiddict-core/tree/maui_winui_sample

[kevinchalet]

Sample updated to use OpenIddict.Client.SystemIntegration (that replaces OpenIddict.Client.Windows).

[kevinchalet]

Branch updated to adopt the final version of OpenIddict.Client.SystemIntegration that was announced here: https://kevinchalet.com/2023/02/27/introducing-system-integration-support-for-the-openiddict-client/

[kevinchalet]

Branch updated to target OpenIddict 5.0.1 and .NET 8.0/MAUI 8.0.3.