Add support for filestorage testing

[phausman]

This PR allows Woodpecker charm to handle filestorage instead of just block storage. The idea is to be able to benchmark Manilla-Ganesha or other RWM file-systems.

The example below triggers fio tests using /mnt/nfs/woodpecker-0 file.

# Tell woodpecker where the files for fio testing should be laid out
juju config woodpecker directories=/mnt/nfs

# Mount a NFS share in woodpecker unit
juju run --application woodpecker "sudo mount -t nfs ${SHARE} /mnt/nfs/"

# Run fio test with a 10GB file
juju run-action woodpecker/0 \
  fio operation=randwrite runtime=60 block-size=4k filesize=10G