Add Kernel Lifecycle State Management Mixin

[Zsailer]

Addresses #763.

This PR introduces a state machine for tracking kernel lifecycle states using a mixin approach that can be easily adopted by any existing kernel manager implementation.

The KernelManagerStateMixin design allows developers to add state tracking to any existing kernel
manager by simply including it in the inheritance chain - no configuration or code changes required.
This opt-in approach means:

• Existing implementations continue working unchanged
• Developers can choose when and where to add state tracking
• Zero risk of breaking current functionality
• Easy to test and evaluate before wider adoption

I intentionally did not add this to the default kernel managers yet, allowing the community to
evaluate the approach first.

Usage

from jupyter_client import KernelManagerStateMixin

class MyKernelManager(KernelManagerStateMixin, ExistingKernelManager):
    pass  # No changes needed to existing logic

manager = MyKernelManager()
manager.start_kernel()
print(manager.is_started)  # True