Support all-squash mount option

[kyungwan-nam]

What would you like to be added:

root-squash is supported by #3562. I want to support all-squash mount option as well.
It makes any user mounted to the JuiceFS to use the uid/gid given all-squash parameter instead of their actual uid/gid.
nfs support all_squash as well as root_squash https://linux.die.net/man/5/exports

Why is this needed:

we use the JuiceFS across hadoop / k8s.
so, I want file's uid/gid to be consistent.
mapping uid/gid is possible in hadoop hadoop_java_sdk/user-permissions. but, it isn't in k8s.
I expect all-squash achieve it in k8s.

[kyungwan-nam]

Pull Request: #5395
could you take a look at this?
thanks in advance!