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>

config.yaml.example

@@ -50,3 +50,13 @@ checks:
     community: "public"
     oid: "1.3.6.1.2.1.1.1.0"  # sysDescr
     interval: 120
+
+  # DNS checks
+  - id: pihole
+    name: "Pi-hole DNS"
+    type: dns
+    host: "192.168.1.10"
+    query: "google.com"
+    record_type: A  # A, AAAA, TXT, MX, etc.
+    timeout: 5
+    interval: 60