Add ecdsa cert support for client through SSH agent

[xRemaLx]

Add ecdsa-sha2-nistp(256|384|521)-cert-v01@openssh.com for client through SSH_AGENT

It is work in case from #910 and #858

I test on my server like this:

const { Client } = require('ssh2');

const conn = new Client();
conn.on('ready', () => {
    console.log('Client :: ready');
    conn.exec('uptime', (err, stream) => {
        if (err) throw err;
        stream.on('close', (code, signal) => {
          console.log('Stream :: close :: code: ' + code + ', signal: ' + signal);
          conn.end();
        }).on('data', (data) => {
          console.log('STDOUT: ' + data);
        }).stderr.on('data', (data) => {
          console.log('STDERR: ' + data);
        });
    });
}).connect({
    host: 'test.server',
    port: 22,
    username: 'xremalx',
    agent: process.env.SSH_AUTH_SOCK,
});

[kobrineli]

@mscdex
Hi! Could you review this, please?