Add CancellationToken Parameter to Methods Returning Task and ValueTask

[brminnick]

Description of Change

This PR updates every method that returns a Task or ValueTask to include a CancellationToken as a parameter.

This ensures that the .NET MAUI Community Toolkit follows Microsoft's recommended async/await Best Practices to always support Task Cancellation.

Requiring a CancellationToken also ensures that analyzer CA2016 ("Forward the 'cancellationToken' parameter methods that take one") correctly alerts developers using our library to propagate their CancellationToken to our APIs.

I've also removed the default values for CancellationToken parameters (eg CancellationToken token = default -> CancellationToken token) . This follows the best-practice recommendation for Libraries to require the developer to make a thoughtful choice as to how to handle cancellation.

Update: Looking at the .NET BCL, their APIs provide default values for CancellationToken. Therefore, we too will provide default values for our CancellationToken parameters (eg Cancellation token = default)).

Example: IDrawingView.GetImageStream

For example, the IDrawingView.GetImageStream method changes from

ValueTask<Stream> GetImageStream(double imageSizeWidth, double imageSizeHeight, Paint background);

becomes

ValueTask<Stream> GetImageStream(double imageSizeWidth, double imageSizeHeight, Paint background, CancellationToken token = default);

Linked Issues

• Fixes Adding a CancellationToken token parameter to ValueTask functions in the Validators. #225

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: Add CancellationToken Parameter to Methods Returning Task and ValueTask MicrosoftDocs/CommunityToolkit#335

Additional information

In the .editorconfig, I've promoted CA1068 to error to ensure that we follow best-practices when adding a CancellationToken as a parameter to methods.

I've added the breaking change This label is used for PRs that include a breaking change label because this modifies existing APIs. I recommend merging this before we publish CommunityToolkit v7.0.0 which contains the existing breaking change targeting .NET 8.

This PR also adds a Timeout to every Unit Test that returns a Task to ensure our Unit Test timeout when a deadlock is encountered.

I've also added do not merge Do not merge this PR label until Unit Tests are added and Documentation is updated.

[pictos]

This is a great idea! I've two comments around the current implementation

[brminnick]

@pictos Ok! This PR is ready for Review (again!).

• Docs are complete Add CancellationToken Parameter to Methods Returning Task and ValueTask MicrosoftDocs/CommunityToolkit#335
• Cancellation Unit Tests written
• Public APIs Include a Default Value for CancellationToken (eg CancellationToken token = default)
• Contributing.md Updated to Include CancellationToken guidelines

I've removed the do not merge Do not merge this PR label