Support Object Storage *prefix* in format command so that we can restrict JuiceFs under a specific prefix

[ericsun2]

What would you like to be added:

juicefs format --storage s3 \
    --bucket https://myjuicefs.s3.us-east-2.amazonaws.com/prefix_to_juicefs/project_01 \
    redis://192.168.1.6/1 \
    myjfs

or

juicefs format --storage s3 \
    --bucket https://myjuicefs.s3.us-east-2.amazonaws.com \
    --prefix prefix_to_juicefs/project_01 \
    redis://192.168.1.6/1 \
    myjfs

Why is this needed:
In some case, we can restrict JuiceFs to operate within a given prefix in the Object Storage. The above S3 example should be applicable to GCS, Azure, ... as well. s3://myjuicefs/prefix_to_juicefs/project_01 will be the root location for the format command to create a volume.

Object Storage ACL can also be applied to the prefix.

[davies]

right now, we use volume name as the prefix, why that is not enough?

[is]

There is indeed a need for this

[1247862674]

I now want to format the S3 prefix (/project/project1/userdata). What do I need to do?