[skygear asset] High entropy link for public access asset

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Description

Currently, private access asset has a 7 days limit, while public access asset has no limit.
However, in some case we want a private asset without time limit. This is usually solved by using a high entropy link with public access, whereas the entropy could serve as "password" purpose.

For example, github do this, note that github objective is anonymized url so might not have high entropy.