HarmonyOS 指定私钥生成SM2秘钥的方法

参考demo：

let base64 = new util.Base64Helper()
let pubkeyBlob = "cRRo55FS5AS/CoifJ2...xqUuOpo3WHEE9AS9cpq8c="
let pubkey = base64.decodeSync(pubkeyBlob)
let pubk: cryptoFramework.DataBlob = { data: pubkey };
let keyGenAlg = "SM2_256";
let sm2Generator = cryptoFramework.createAsyKeyGenerator('SM2_256');
let keyPair = await sm2Generator.convertKey(null, pubk);
let signData = await signMessagePromise(keyPair.priKey);