feat(core): Allow idle and deep sleep timeouts to be set at runtime

[ReFil]

This adds functions to set the idle and deep sleep timeouts, the new timeouts are persisted in settings across reboots of the board

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.

[Nick-Munnich]

Compiler probably already puts this inline, but this feels like it should be inline to me. I don't know enough about the specifics of what it would do for certain though, feel free to ignore.

[ReFil]

Ngl I don't know much about inlining functions, I modelled this after the endpoints code and I didn't recall it being inlined there

[Nick-Munnich]

Suggested change

	void zmk_activity_set_idle_timeout(uint32_t timeout);
	#if IS_ENABLED(CONFIG_ZMK_SLEEP)
	uint32_t zmk_activity_get_sleep_timeout(void);
	void zmk_activity_set_sleep_timeout(uint32_t timeout);
	int zmk_activity_set_idle_timeout(uint32_t timeout);
	#if IS_ENABLED(CONFIG_ZMK_SLEEP)
	uint32_t zmk_activity_get_sleep_timeout(void);
	int zmk_activity_set_sleep_timeout(uint32_t timeout);

These feel like they should return the value from activity_save_timeouts in case of error messages.