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/self.go

@@ -0,0 +1,26 @@
+package auth
+
+import (
+	"context"
+
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+// self returns current user.
+//
+// You can use tg.User.Bot to check whether current user is bot.
+func (c *Client) self(ctx context.Context) (*tg.User, error) {
+	users, err := c.api.UsersGetUsers(ctx, []tg.InputUserClass{&tg.InputUserSelf{}})
+	if err != nil {
+		return nil, err
+	}
+
+	user, ok := tg.UserClassArray(users).FirstAsNotEmpty()
+	if !ok {
+		return nil, errors.Errorf("users response count: %v", users)
+	}
+
+	return user, nil
+}