Allow behavior locality to be configurable

[xudongzheng]

This would enable a number of use cases particularly when using a dongle.

• &reset and &bootloader currently default to BEHAVIOR_LOCALITY_EVENT_SOURCE. BEHAVIOR_LOCALITY_CENTRAL would allow user to reset or trigger bootloader on dongle instead of peripheral half.
• &ext_power, &rgb_ug, and &bl default to BEHAVIOR_LOCALITY_GLOBAL. BEHAVIOR_LOCALITY_CENTRAL would allow user to manipulate LEDs connected to dongle (where power consumption is not a concern) without affecting keyboard halves.

One option is to make it a DeviceTree attribute, which will allow users to define separate keys that are local/global/source.

Also important to think about how CONFIG_ZMK_RGB_UNDERGLOW_EXT_POWER should behave. Most logical would be for it to do whatever the default &ext_vcc does.