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/crypto/public_keys.go

@@ -0,0 +1,48 @@
+package crypto
+
+import (
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+
+	"github.com/go-faster/errors"
+)
+
+// ParseRSAPublicKeys parses data as list of PEM-encdoed public keys.
+func ParseRSAPublicKeys(data []byte) ([]*rsa.PublicKey, error) {
+	var keys []*rsa.PublicKey
+
+	for {
+		block, rest := pem.Decode(data)
+		if block == nil {
+			break
+		}
+
+		key, err := ParseRSA(block.Bytes)
+		if err != nil {
+			return nil, errors.Wrap(err, "parse RSA from PEM")
+		}
+
+		keys = append(keys, key)
+		data = rest
+	}
+
+	return keys, nil
+}
+
+// ParseRSA parses data RSA key in PKCS1 or PKIX forms.
+func ParseRSA(data []byte) (*rsa.PublicKey, error) {
+	key, err := x509.ParsePKCS1PublicKey(data)
+	if err == nil {
+		return key, nil
+	}
+	k, err := x509.ParsePKIXPublicKey(data)
+	if err != nil {
+		return nil, err
+	}
+	publicKey, ok := k.(*rsa.PublicKey)
+	if !ok {
+		return nil, errors.Errorf("parsed unexpected key type %T", k)
+	}
+	return publicKey, nil
+}