sd-dhcp-client: allow getting/setting the client ID

The client identifier can be in many different formats, not just
the one that systemd creates from the Ethernet MAC address.  Non-
ethernet interfaces may have different client IDs formats.  Users
may also have custom client IDs that the wish to use to preserve
lease options delivered by servers configured with the existing
client ID.
sd-dhcp-lease: load/save client ID

The lease is usually tied to the client ID, so users of the
lease may want to know what client ID it was acquired with.