feat(split): Increase split interval during idle

[carrefinho]

Increase BLE split connection interval when idle to significantly reduce central power consumption. There's a brief period (about 0.6-1s) where the chances of having misordered keypresses are higher when coming out of idle.

Based on #682, updated for current main and fixes issue of finding an extra invalid connection when trying to update.

• Add central_idle.c listener that increases connection interval when activity state changes to idle
• Add idle connection parameter Kconfig options under ZMK_SPLIT_BLE_PREF_IDLE_*
• Temporary: use Zephyr branch with a cherry-picked Bluetooth controller fix for crashing when updating connection parameters on the peripheral. Cherry pick of Bluetooth controller peripheral conn update fix zephyr#51

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.