feat: add coder connect exists hidden subcommand (#17418)

Adds a new hidden subcommand `coder connect exists <hostname>` that checks if the name exists via Coder Connect. This will be used in SSH config to match only if Coder Connect is unavailable for the hostname in question, so that the SSH client will directly dial the workspace over an existing Coder Connect tunnel.

Also refactors the way we inject a test DNS resolver into the lookup functions so that we can test from outside the `workspacesdk` package.

cli/connect.go

@@ -0,0 +1,47 @@
+package cli
+
+import (
+	"github.com/coder/serpent"
+
+	"github.com/coder/coder/v2/codersdk/workspacesdk"
+)
+
+func (r *RootCmd) connectCmd() *serpent.Command {
+	cmd := &serpent.Command{
+		Use:   "connect",
+		Short: "Commands related to Coder Connect (OS-level tunneled connection to workspaces).",
+		Handler: func(i *serpent.Invocation) error {
+			return i.Command.HelpHandler(i)
+		},
+		Hidden: true,
+		Children: []*serpent.Command{
+			r.existsCmd(),
+		},
+	}
+	return cmd
+}
+
+func (*RootCmd) existsCmd() *serpent.Command {
+	cmd := &serpent.Command{
+		Use:   "exists <hostname>",
+		Short: "Checks if the given hostname exists via Coder Connect.",
+		Long: "This command is designed to be used in scripts to check if the given hostname exists via Coder " +
+			"Connect. It prints no output. It returns exit code 0 if it does exist and code 1 if it does not.",
+		Middleware: serpent.Chain(
+			serpent.RequireNArgs(1),
+		),
+		Handler: func(inv *serpent.Invocation) error {
+			hostname := inv.Args[0]
+			exists, err := workspacesdk.ExistsViaCoderConnect(inv.Context(), hostname)
+			if err != nil {
+				return err
+			}
+			if !exists {
+				// we don't want to print any output, since this command is designed to be a check in scripts / SSH config.
+				return ErrSilent
+			}
+			return nil
+		},
+	}
+	return cmd
+}