WIP: Add mount command

[aawsome]

This PR adds the mount command to rustic to access snapshot contents as a read-only filesystem.

As for the webdav command, there are following options:

• Access to a concrete snapshot/path, e.g. rustic mount /mnt 37a63e5b:/my/path.
• Access to all snapshots (maybe restricted by filters) by using templates to define a virtual directory structure where snapshots are located. Example: rustic mount /mnt --path-template "[{hostname}]/[{label}]/{time}" --time-template "%Y-%m-%d_%H-%M-%S" (these are also defined as default). Note that for all dirs containing only snapshots, also a latest entry is generated. latest and identical subsequent snapshots are symlinks when using mount.

Depends on rustic-rs/rustic_core#106 and is based on #966

This PR uses fuse_mt which is not optimal as it introduces some overhead (e.g. needs to save whole Paths in-memory).

closes #971

TODO:

• update Cargo.toml once feat: add FuseFs rustic_core#156 has been merged into rustic_core -> no longer needed
• add mount options, see discussion below
• Adjust CI-Pipeline to use fues libraries