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/mtproxy/obfuscated2/server.go

@@ -0,0 +1,42 @@
+package obfuscated2
+
+import (
+	"encoding/binary"
+	"io"
+)
+
+// Metadata represents metadata received from header.
+type Metadata struct {
+	Protocol [4]byte
+	DC       uint16
+}
+
+// Accept creates new io.ReadWriter for server-side deobfuscation.
+func Accept(conn io.ReadWriter, secret []byte) (io.ReadWriter, Metadata, error) {
+	var (
+		buf  = make([]byte, 64)
+		meta Metadata
+	)
+	if _, err := io.ReadFull(conn, buf); err != nil {
+		return nil, meta, err
+	}
+
+	var k keys
+	if err := k.createStreams(buf, secret); err != nil {
+		return nil, meta, err
+	}
+	// Swap to match client's streams.
+	k.encrypt, k.decrypt = k.decrypt, k.encrypt
+
+	var decrypted [64]byte
+	k.decrypt.XORKeyStream(decrypted[:], buf)
+
+	copy(meta.Protocol[:], decrypted[56:60])
+	meta.DC = binary.LittleEndian.Uint16(decrypted[60:62])
+
+	return &Obfuscated2{
+		rand: nil, // Used only in Handshake.
+		conn: conn,
+		keys: k,
+	}, meta, nil
+}