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/telegram/auth/signup.go

@@ -0,0 +1,37 @@
+package auth
+
+import (
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+// SignUpRequired means that log in failed because corresponding account
+// does not exist, so sign up is required.
+type SignUpRequired struct {
+	TermsOfService tg.HelpTermsOfService
+}
+
+// Is returns true if err is SignUpRequired.
+func (s *SignUpRequired) Is(err error) bool {
+	_, ok := err.(*SignUpRequired)
+	return ok
+}
+
+func (s *SignUpRequired) Error() string {
+	return "account with provided number does not exist (sign up required)"
+}
+
+// checkResult checks that `a` is *tg.AuthAuthorization and returns authorization result or error.
+func checkResult(a tg.AuthAuthorizationClass) (*tg.AuthAuthorization, error) {
+	switch a := a.(type) {
+	case *tg.AuthAuthorization:
+		return a, nil // ok
+	case *tg.AuthAuthorizationSignUpRequired:
+		return nil, &SignUpRequired{
+			TermsOfService: a.TermsOfService,
+		}
+	default:
+		return nil, errors.Errorf("got unexpected response %T", a)
+	}
+}