Replace futures with async publisher

[defagos]

Description

Currently we use futures to perform async work. Tasks started this way will never be properly cancellable. To improve this behavior this PR introduces a publisher whose subscription manages a cancellable task.

Changes made

• Add AsyncPublisher, a publisher that performs an async unit of work.
• Improve existing publisher implementations (no change in behavior, only code moved around).
• Require Swift 6.0 for compilation (the async publisher uses typed throws internally).
• Update all futures wrapping async code to use the new AsyncPublisher.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).