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/transport/codec.go

@@ -0,0 +1,19 @@
+package transport
+
+import (
+	"io"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/bin"
+)
+
+// Codec is MTProto transport protocol encoding abstraction.
+type Codec interface {
+	// WriteHeader sends protocol tag if needed.
+	WriteHeader(w io.Writer) error
+	// ReadHeader reads protocol tag if needed.
+	ReadHeader(r io.Reader) error
+	// Write encode to writer message from given buffer.
+	Write(w io.Writer, b *bin.Buffer) error
+	// Read fills buffer with received message.
+	Read(r io.Reader, b *bin.Buffer) error
+}