tailcfg: pointerify RegisterRequest.Auth, omitemptify RegisterResponseAuth

We were storing server-side lots of:

    "Auth":{"Provider":"","LoginName":"","Oauth2Token":null,"AuthKey":""},

That was about 7% of our total storage of pending RegisterRequest
bodies.

Updates tailscale/corp#19327

Change-Id: Ib73842759a2b303ff5fe4c052a76baea0d68ae7d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg.go

@@ -1069,10 +1069,13 @@ func (st SignatureType) String() string {
 // in response to a RegisterRequest.
 type RegisterResponseAuth struct {
 	_ structs.Incomparable
+
 	// One of Provider/LoginName, Oauth2Token, or AuthKey is set.
-	Provider, LoginName string
-	Oauth2Token         *Oauth2Token
-	AuthKey             string
+
+	Provider    string       `json:",omitempty"`
+	LoginName   string       `json:",omitempty"`
+	Oauth2Token *Oauth2Token `json:",omitempty"`
+	AuthKey     string       `json:",omitempty"`
 }
 
 // RegisterRequest is sent by a client to register the key for a node.
@@ -1093,7 +1096,7 @@ type RegisterRequest struct {
 	NodeKey    key.NodePublic
 	OldNodeKey key.NodePublic
 	NLKey      key.NLPublic
-	Auth       RegisterResponseAuth
+	Auth       *RegisterResponseAuth `json:",omitempty"`
 	// Expiry optionally specifies the requested key expiry.
 	// The server policy may override.
 	// As a special case, if Expiry is in the past and NodeKey is