Asset objects’s URL expire day should be explicit

[chpapa]

Asset object have a URL parameter, which could be expired. But the expire date is not explicit in the object. We should have a parameter in the Asset object to indicate when the URL expire.

It is useful when developer have to persist the object / use it for a longer time.

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

Expected Results

Something like assetObject.url_expire_at

Actual Results

Steps to reproduce