feat: move service to background

[RohitKushvaha01]

This PR introduces a new background executor, making it possible to run executors without starting a foreground service or showing a notification.

By default, executors continue to run in foreground mode (unchanged behavior). The new background executor allows running tasks in background mode.

New Background Executor

// Foreground executor (default – runs as a foreground service with notification)
Executor.start('sh', callback);

// Background executor (runs without a foreground service or notification)
Executor.BackgroundExecutor.start('sh', callback);

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New apis for the default Executor

⚠️ Note:
When running in background mode, the service has a higher chance of being killed by the system after the app exits.

Service Behavior

• The executor service is shared across all Executor instances.
• Switching between foreground and background affects all active executors.
• If the service is not running, starting either executor will launch it automatically.

Related APIs (secondary)

Two internal APIs are added to move the shared service between foreground and background modes:

• Executor.moveToBackground()
• Executor.moveToForeground()

Closes #1666