hadoop sdk: verify kerberos authenticated user

[kyungwan-nam]

What would you like to be added:

I want all file's UID to be consistent in our kerberos Hadoop cluster using mapping username/UID for all kerberos hadoop users.
Currently, The JuiceFS Hadoop SDK authenticate as the OS user if there’s no kerberos credentials. so, can use the JuiceFS as unknown user not in the mapping username/UID. It makes it difficult to ensure consistent.

Why is this needed:

I'd like to add a new property juicefs.verify-kerberos-user (default: false) for JuiceFS Hadoop SDK.
It allows only for kerberos-authenticated user If juicefs.verify-kerberos-user is true

[kyungwan-nam]

Requires kerberos credentials if juicefs.verify-kerberos-user is true. there's no side effect if false(default)
Could you take a look at PR: #5554