api: FileRestore: make use of file-restores and guis timeout mechanism

api: FileRestore: make use of file-restores and guis timeout mechanism

file-restore has a 'timeout' parameter and if that is exceeded, returns
an error with the http code 503 Service Unavailable

When the web ui encounters such an error, it retries the listing a few
times before giving up.

To make use of these, add the 'timeout' parameter to the new
'extraParams' of 'file_restore_list'.

25 seconds are chosen because it's under pveproxy 30s limit, with a bit
of overhead to spare for the rest of the api call, like json decoding,
forking, access control checks, etc.

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