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/tgtest/handler.go

@@ -0,0 +1,37 @@
+package tgtest
+
+import (
+	"context"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/bin"
+)
+
+// Request represents MTProto RPC request structure.
+type Request struct {
+	// DC ID from server structure.
+	// Used to make handler less stateful.
+	DC int
+	// Session is a user session.
+	Session Session
+	// MsgID is a message ID of RPC request.
+	MsgID int64
+	// Buf contains RPC request
+	Buf *bin.Buffer
+	// RequestCtx is a request context.
+	RequestCtx context.Context
+}
+
+// Handler is a RPC request handler.
+type Handler interface {
+	OnMessage(server *Server, req *Request) error
+}
+
+var _ Handler = HandlerFunc(nil)
+
+// HandlerFunc is functional adapter for Handler.OnMessage method.
+type HandlerFunc func(server *Server, req *Request) error
+
+// OnMessage implements Handler.
+func (h HandlerFunc) OnMessage(server *Server, req *Request) error {
+	return h(server, req)
+}