Improve asset getSigner import

[rickmak]

Current way of import getSigner is as follow:

const getSigner = require('skygear-core/dist/cloud/asset').getSigner;

• Skygear SDK Date/Version: 1.0.0
• JS runtime: node
• Skygear Server Date/Version: 1.0.0
• [ ] Is this a regression?
• [ ] Attached logs, screenshots

Expected Results

import getSigner from 'skygear/cloud'; const getSigner = require('skygear/cloud').getSigner;