Add support for custom domain to `netlify dev`

[ericbaukhages]

Which problem is this feature request solving?

We'd like to be able to use a custom domain with local development, instead of localhost, with netlify dev.

We need a custom local domain for various security settings on supporting services. Additionally, the HTTPS certificate we'd like to use for local development is attached to said custom domain.

Describe the solution you'd like

Ideally, there would be a setting for custom local domain, to replace localhost. This setting would exist alongside the settings for port or targetPort. Like those settings, these setting could be set from:

• The command line for netlify dev, e.g. netlify dev --domain custom.dev

• The netlify.toml section for [dev], e.g.:

  [dev]
    domain = "custom.dev"
    port = "8888"
  

Pull request (optional)

• [X] I can submit a pull request.