introduce a CryptMode enum

introduce a CryptMode enum

This also replaces the recently introduced --encryption
parameter on the client with a --crypt-mode parameter.

This can be "none", "encrypt" or "sign-only".

Note that this introduces various changes in the API types
which previously did not take the above distinction into
account properly:

Both `BackupContent` and the manifest's `FileInfo`:
    lose `encryption: Option<bool>`
    gain `crypt_mode: Option<CryptMode>`

Within the backup manifest itself, the "crypt-mode" property
will always be set.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>