DhcService cleanup

[bmingles]

The DhcService has been around since early POC and could use some cleanup:

• Combine DhService + DhcService since DheService is independent. No longer need a base class, and it makes it harder to reason about the code
• Remove vscode.window dependency from DhcService and use a "command" pattern modeled after DheService
• Replace DhcServiceFactory with static method on DhcService (or refactor DheService to match)