Add DNS checker

Checks DNS server availability via ping then performs configurable
DNS query (record_type: A, AAAA, TXT, MX, etc.).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

checkers/__init__.py

@@ -1,9 +1,11 @@
 from .ping import PingChecker
 from .http import HttpChecker
 from .snmp import SnmpChecker
+from .dns import DnsChecker
 
 CHECKERS = {
     "ping": PingChecker,
     "http": HttpChecker,
     "snmp": SnmpChecker,
+    "dns": DnsChecker,
 }