resolved: added serve stale feature implementation of RFC 8767

serve stale feature to keep the DNS resource records beyond TTL to return them as stale records in case of upstream server is not reachable or returns negative response.
SD_RESOLVED_NO_STALE flag has been added to disable serving stale records via dbus.
added serve stale test cases to TEST-75-RESOLVED
Fixes: #21815

shell-completion/bash/resolvectl

@@ -37,7 +37,7 @@ _resolvectl() {
         [STANDALONE]='-h --help --version -4 -6 --legend=no --cname=no
                       --validate=no --synthesize=no --cache=no --zone=no
                       --trust-anchor=no --network=no --service-address=no
-                      --service-txt=no --search=no --no-pager'
+                      --service-txt=no --search=no --stale-data=no --no-pager'
         [ARG]='-t --type -c --class -i --interface -p --protocol --raw'
     )
     local -A VERBS=(