Relocate the kea dhcp op-mode functions to kea helper functions in vyos.kea. This allows the functions to be reused by other scripts, not just op-mode wrappers.

This also moves the source of truth for the op-mode commands to the actual running kea instance, rather than VyOS config path.

This task is based on feedback received in PR (https://github.com/vyos/vyos-1x/pull/4237#discussion_r1912516708) for T6948.