dhcp: T7052: Refactor kea dhcp op-mode functions to vyos.kea

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

Also, apply some minor code cleanup and make some
of the mappings consistent across the functions.