OLMKit: Expose PK private key length

xcode/OLMKit/OLMPkDecryption.h

@@ -59,6 +59,13 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (NSString *)decryptMessage:(OLMPkMessage*)message error:(NSError* _Nullable *)error;
 
+/**
+ Private key length.
+
+ @return the length in bytes.
+ */
++ (NSUInteger)privateKeyLength;
+
 @end
 
 NS_ASSUME_NONNULL_END

xcode/OLMKit/OLMPkDecryption.m

@@ -189,6 +189,10 @@
     return plaintext;
 }
 
++ (NSUInteger)privateKeyLength {
+    return olm_pk_private_key_length();
+}
+
 #pragma mark OLMSerializable
 
 /** Initializes from encrypted serialized data. Will throw error if invalid key or invalid base64. */