fix(rest): validation of httpAgent config

[kobenguyent]

Motivation/Description of the PR

• fix the validation of httpAgent config. we could now pass ca, instead of key/cert.

{
  helpers: {
    REST: {
      endpoint: 'http://site.com/api',
      prettyPrintJson: true,
      httpAgent: {
         ca: fs.readFileSync(__dirname + '/path/to/ca.pem'),
         rejectUnauthorized: false,
         keepAlive: true
      }
    }
  }
}

Applicable helpers:

• REST

Type of change

• 🐛 Bug fix

Checklist:

• Tests have been added
• Documentation has been added (Run npm run docs)
• Lint checking (Run npm run lint)
• Local tests are passed (Run npm test)