src: add default value for RSACipherConfig mode field

Using default init of enum is UB
Refs: #56693

src/crypto/crypto_rsa.h

@@ -77,7 +77,7 @@ struct RSAKeyExportTraits final {
 using RSAKeyExportJob = KeyExportJob<RSAKeyExportTraits>;
 
 struct RSACipherConfig final : public MemoryRetainer {
-  CryptoJobMode mode;
+  CryptoJobMode mode = kCryptoJobAsync;
   ByteSource label;
   int padding = 0;
   const EVP_MD* digest = nullptr;