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/testutil/env.go

@@ -0,0 +1,12 @@
+package testutil
+
+import (
+	"testing"
+)
+
+// SetEnv sets environment variable and sets delete callback
+// to unset variable after test.
+func SetEnv(t *testing.T, k, v string) {
+	// Set envs.
+	t.Setenv(k, v)
+}