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/exchange/key.go

@@ -0,0 +1,46 @@
+package exchange
+
+import (
+	"crypto/rsa"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/crypto"
+)
+
+// PublicKey is a public Telegram server key.
+type PublicKey struct {
+	// RSA public key.
+	RSA *rsa.PublicKey
+}
+
+// Zero denotes that current PublicKey is zero value.
+func (k PublicKey) Zero() bool {
+	return k.RSA == nil
+}
+
+// Fingerprint computes key fingerprint.
+func (k PublicKey) Fingerprint() int64 {
+	return crypto.RSAFingerprint(k.RSA)
+}
+
+// PrivateKey is a private Telegram server key.
+type PrivateKey struct {
+	// RSA private key.
+	RSA *rsa.PrivateKey
+}
+
+// Zero denotes that current PublicKey is zero value.
+func (k PrivateKey) Zero() bool {
+	return k.RSA == nil
+}
+
+// Fingerprint computes key fingerprint.
+func (k PrivateKey) Fingerprint() int64 {
+	return crypto.RSAFingerprint(&k.RSA.PublicKey)
+}
+
+// Public returns PublicKey of this PrivateKey pair.
+func (k PrivateKey) Public() PublicKey {
+	return PublicKey{
+		RSA: &k.RSA.PublicKey,
+	}
+}