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/rpc/external.go

@@ -0,0 +1,23 @@
+package rpc
+
+import (
+	"context"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/bin"
+)
+
+// Send is a function that sends requests to the server.
+type Send func(ctx context.Context, msgID int64, seqNo int32, in bin.Encoder) error
+
+// NopSend does nothing.
+func NopSend(context.Context, int64, int32, bin.Encoder) error { return nil }
+
+var _ Send = NopSend
+
+// DropHandler handles drop rpc requests.
+type DropHandler func(req Request) error
+
+// NopDrop does nothing.
+func NopDrop(Request) error { return nil }
+
+var _ DropHandler = NopDrop