ipn/ipnlocal: add c2n method to get SSH username candidates

For control to fetch a list of Tailscale SSH username candidates to
filter against the Tailnet's SSH policy to present some valid
candidates to a user.

Updates #3802
Updates tailscale/corp#7007

Change-Id: I3dce57b7a35e66891d5e5572e13ae6ef3c898498
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -5,8 +5,11 @@
 package ipnlocal
 
 import (
+	"encoding/json"
 	"io"
 	"net/http"
+
+	"tailscale.com/tailcfg"
 )
 
 func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
@@ -15,6 +18,21 @@ func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
 		// Test handler.
 		body, _ := io.ReadAll(r.Body)
 		w.Write(body)
+	case "/ssh/usernames":
+		var req tailcfg.C2NSSHUsernamesRequest
+		if r.Method == "POST" {
+			if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+				http.Error(w, err.Error(), http.StatusBadRequest)
+				return
+			}
+		}
+		res, err := b.getSSHUsernames(&req)
+		if err != nil {
+			http.Error(w, err.Error(), 500)
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(res)
 	default:
 		http.Error(w, "unknown c2n path", http.StatusBadRequest)
 	}