certificate.exportPublicKey(spkac)
spkac<string> | <Buffer> | <TypedArray> | <DataView>- 返回: <Buffer> 
spkac数据结构的公钥组件,包括公钥和挑战。 
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// 打印: the public key as <Buffer ...>spkac<string> | <Buffer> | <TypedArray> | <DataView>- Returns: <Buffer> The public key component of the 
spkacdata structure, which includes a public key and a challenge. 
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>