move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/telegram/auth.go

@@ -0,0 +1,23 @@
+package telegram
+
+import (
+	"go.mau.fi/mautrix-telegram/pkg/gotd/telegram/auth"
+	"go.mau.fi/mautrix-telegram/pkg/gotd/telegram/auth/qrlogin"
+)
+
+// Auth returns auth client.
+func (c *Client) Auth() *auth.Client {
+	return auth.NewClient(
+		c.tg, c.rand, c.appID, c.appHash,
+	)
+}
+
+// QR returns QR login helper.
+func (c *Client) QR() qrlogin.QR {
+	return qrlogin.NewQR(
+		c.tg,
+		c.appID,
+		c.appHash,
+		qrlogin.Options{Migrate: c.MigrateTo},
+	)
+}