feat: an HA pair's DHCP peers and config sync as facts (4.11.0)

get_dhcp_ha (declared on DhcpServerMixin, optional) is what a DHCP
server's own HA configuration says: on or off, this server's peer name,
each peer's name, role as the server words it, and HA URL.
ConfigSyncMixin.get_config_sync is where a host pushes its configuration
and for which services, by the names get_services reports; sync items
without a service name stay visible under `unmapped`. A driver mixes it
in, no role promises it. Both raise when they could not read.

What the facts mean for a pair -- hot standby, which side to write to --
is the caller's (NetOrk/netork#1125).

README.md

@@ -128,6 +128,17 @@ A mechanism it could not read completely is `None` ("did not look"), a host with
 `[]`. The command is about 4 kB, so a driver sends it on an exec channel
 (`_run_virtual_ips_command(command, privileged=)`), not typed into a shell.
 
+`get_dhcp_ha` (declared on `DhcpServerMixin`, optional) is what a DHCP server's own HA
+configuration says: whether HA is on, which peer this server is, each peer's name, role --
+as the server words it, not a shared vocabulary -- and, where the server has one, the URL
+its HA listener answers on. It is configuration, not state: whether the pair is in hot
+standby right now is not in it. `ConfigSyncMixin` (`get_config_sync`) is where a host
+pushes its configuration (`peer`, None for no one) and for which services, by the names
+`get_services` reports; sync items the driver has no service name for stay visible under
+`unmapped`. A driver mixes it in -- no role promises it. Both getters raise when they could
+not read; neither answers "off" for "could not ask". What the facts mean for a pair (hot
+standby, which side to write to) is the caller's to work out.
+
 A firewall's CARP addresses are a third mechanism, `carp`: a firewall driver reads them from
 its own API and reports them under the reading's `carp` key (`vip_manager` and `keepalived`
 then None). What a CARP state means is protocol knowledge and stays in that driver. The