derive/impl and use Default for some structs

and revamp HttpClientOptions with two constructors for the common use
cases

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

src/backup/prune.rs

@@ -67,6 +67,7 @@ fn remove_incomplete_snapshots(
     }
 }
 
+#[derive(Default)]
 pub struct PruneOptions {
     pub keep_last: Option<u64>,
     pub keep_hourly: Option<u64>,