api: Add optional parameters `since` and `until` for timestamp filter

api: Add optional parameters `since` and `until` for timestamp filter

The optional unix epoch timestamps parameters `since` and `until` are introduced
in order to filter firewall logs files. If one of these flags is set, also
rotated logfiles are included. This is handled in the `dump_fw_logfile` helper
function. Filtering is now performed based on a callback function passed to
`dump_fw_logfile`.

This patch depends on the corresponding patch in the pve-common repository.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
[w.bumiller@proxmox.com: fixup 'continue' -> 'next']
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>