Heartbeat request support

[Baraujo25]

This pull request introduces support for periodic "heartbeat" requests during streaming playback, enabling more realistic simulation of client behavior (such as analytics beacons or session keep-alives) without the overhead of additional threads. The main changes add a StreamingSliceCoordinator for thread-local coordination between the controller and sampler, a PlaybackSession to persist playback state across slices, and integrate these into the streaming sampler logic. Documentation is updated to explain how to use the new Streaming Parallel Controller.

New streaming parallel playback and heartbeat support:

• Added a new StreamingSliceCoordinator class to coordinate time-sliced playback and heartbeats within a single thread, allowing the sampler to yield control at slice boundaries and resume playback without restarting.
• Introduced a PlaybackSession class to persist protocol-specific playback state across slices, preventing unnecessary re-downloads of manifests/playlists when resuming.

Integration with streaming sampler:

• Updated VideoStreamingSampler and HlsSampler to use the StreamingSliceCoordinator for yielding, session management, and error handling, and to clear playback sessions on errors or interrupts.

Documentation:

• Expanded the README.md with a detailed section on running periodic requests during playback, including setup instructions, configuration options, and limitations of the Streaming Parallel Controller.

Minor refactoring and formatting:

• Cleaned up code formatting and improved property handling in VideoStreamingSampler.
  These changes together enable efficient and realistic simulation of streaming clients that send periodic background requests during playback, with clear documentation for users.