Add support for passing FUSE file descriptors as mount point

[unexge]

Description of change

fuser v0.15.0 added support for creating a Session from existing FUSE file descriptor (via Session::from_fd). This PR adds this support to Mountpoint. It allows passing FUSE file descriptor as mount point in the form of /dev/fd/{fd}.

An example usage of this feature can be seen with a helper Go script, mounthelper.go:

$ go build mounthelper.go
$ sudo /sbin/setcap 'cap_sys_admin=ep' ./mounthelper # `mount` syscall requires `CAP_SYS_ADMIN`, alternatively, `mounthelper` can be run as root
$ ./mounthelper -mountpoint /tmp/mountpoint -bucket bucketname
bucket bucketname is mounted at /dev/fd/3
2024/11/07 17:23:42 Filesystem mounted, waiting for ctrl+c signal to terminate 

$ # in a different terminal session
$ echo "Hello at `date`" > /tmp/mountpoint/helloworld
$ cat /tmp/mountpoint/helloworld
Hello at Thu Nov  7 17:32:33 UTC 2024
$ rm /tmp/mountpoint/helloworld
$ cat /tmp/mountpoint/helloworld
cat: /tmp/mountpoint/helloworld: No such file or directory

Relevant issues: This PR resurrects a previous PR to add this feature: #537

Does this change impact existing behavior?

Shouldn't affect any existing behavior as we had an “is directory?” check for passed mount points before, and it shouldn't have been possible to pass a file descriptor as a mount point prior to this change.

Does this change need a changelog entry in any of the crates?

Updated CHANGELOG for mountpoint-s3.

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