Import dhcp in the executor to avoid blocking the event loop (#111589)

Discovery integrations load a lot earlier when many modules
are not yet loaded so they have a lot less overlapping deps

dhcp uses scapy which is large to import. I hope one
day to replace it with something more lightweight

homeassistant/components/dhcp/manifest.json

@@ -3,6 +3,7 @@
   "name": "DHCP Discovery",
   "codeowners": ["@bdraco"],
   "documentation": "https://www.home-assistant.io/integrations/dhcp",
+  "import_executor": true,
   "integration_type": "system",
   "iot_class": "local_push",
   "loggers": [