Import dhcp in the executor to avoid blocking the event loop

[bdraco]

Proposed change

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

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
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Additional information

• This PR fixes or closes issue: fixes #
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