feat: Async indicator variants (proposal only)

[DaveSkender]

Experiment only. Considering adding indicator variants that return Task for asynchronous operations, since it may better support some additional use cases.

Generally, I'm not sold on adding these because they do not perform as well and aren't easily upgraded to support chaining and possibly streaming + people can Task.Run() to this themselves if needed.

I'll revisit this after v3 streaming features; however, opening this for feedback.

Conclusion: this is over engineered for static methods, users can simply use standard Task.FromResult<TResult>(TResult) if they want to return it in an otherwise async method.

[github-actions]

✔️ Tests 720 / 720 - passed in 7.5s
✔️ Coverage 96.46% - passed with 95% threshold
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✏️ updated for commit 92443f3