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/ping.go

@@ -0,0 +1,10 @@
+package telegram
+
+import (
+	"context"
+)
+
+// Ping sends low level ping request to Telegram server.
+func (c *Client) Ping(ctx context.Context) error {
+	return c.conn.Ping(ctx)
+}