Implement periodic CPU count detection (e.g., via syscalls or OS notifications).
See https://jira.mariadb.org/browse/MDEV-36194 for more information on use case.

Add system variables:
*thread_pool_auto_scale_method=

(default: OFF).

1. thread_pool_auto_scale_factor=N (default: 1).
2. Set thread_pool_size = CPU_count * thread_pool_auto_scale_factor when CPU_count mode is enabled.
3. Reject manual SET GLOBAL thread_pool_size when auto-scaling is active.
4. Make polling frequency configurable (e.g., once per second, per minute), defaulting to once per minute.

Acceptance Criteria

1. thread_pool_auto_scale_method=CPU_count activates CPU-based scaling.
2. Example: CPU_count=8, thread_pool_auto_scale_factor=2, thread_pool_size=16.
3. Manual SET GLOBAL thread_pool_size fails with an error when auto-scaling is enabled.
4. Polling frequency defaults to once per minute and is configurable.
5. Ensure the system variables are manually configurable