config: add JobState helper

this is intended to be a generic helper to (de)serialize job states
(e.g., sync, verify, and so on)

writes a json file into '/var/lib/proxmox-backup/jobstates/TYPE-ID.json'

the api creates the directory with the correct permissions, like
the rrd directory

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/bin/proxmox-backup-api.rs

@@ -37,6 +37,7 @@ async fn run() -> Result<(), Error> {
     config::update_self_signed_cert(false)?;
 
     proxmox_backup::rrd::create_rrdb_dir()?;
+    proxmox_backup::config::jobstate::create_jobstate_dir()?;
 
     if let Err(err) = generate_auth_key() {
         bail!("unable to generate auth key - {}", err);