all: add means to set device posture attributes from node

Updates tailscale/corp#24690
Updates #4077

Change-Id: I05fe799beb1d2a71d1ec3ae08744cc68bcadae2a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -6408,6 +6408,20 @@ func (b *LocalBackend) SetExpirySooner(ctx context.Context, expiry time.Time) er
 	return cc.SetExpirySooner(ctx, expiry)
 }
 
+// SetDeviceAttrs does a synchronous call to the control plane to update
+// the node's attributes.
+//
+// See docs on [tailcfg.SetDeviceAttributesRequest] for background.
+func (b *LocalBackend) SetDeviceAttrs(ctx context.Context, attrs tailcfg.AttrUpdate) error {
+	b.mu.Lock()
+	cc := b.ccAuto
+	b.mu.Unlock()
+	if cc == nil {
+		return errors.New("not running")
+	}
+	return cc.SetDeviceAttrs(ctx, attrs)
+}
+
 // exitNodeCanProxyDNS reports the DoH base URL ("http://foo/dns-query") without query parameters
 // to exitNodeID's DoH service, if available.
 //