feat: add httpsAgent config for keepalive on https request

[ErosZy]

Checklist

• [x] npm test passes
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message follows commit guidelines

Affected core subsystem(s)

add httpsAgent for keepalive requesting

Description of change

according to the doc of egg, we should use httpsAgent to disable the https keepalive, the agent field only set the http keepalive configuration. https://www.eggjs.org/zh-CN/core/httpclient#httpsagent-httpsagent