Object storage support external plugin

[myml]

What would you like to be added: Storage support external plugin, just like

juicefs format --storage juicefs-plugin-xxx.so --bucket xxx sqlite3://myjfs.db myjfs

Here's an example of how to do it with the go plugin https://github.com/juicedata/juicefs/pull/1263.

Why is this needed:

Juicefs now supports many standard protocol storage systems, such as file, webdav and s3. plugin allows jusefs to support more private internal storage systems and does not pollute the juicefs repository.