close #4513: ui: backup job: add tab for advanced options

close #4513: ui: backup job: add tab for advanced options

pigz is not exposed, because it only works after manually installing
the pigz package.

ionice is not exposed, because it only works in combination with the
BFQ scheduler and even then not in all cases (only affects the
compressor when doing snapshot/suspend mode backup of a VM).

The pbs-entries-max performance option is not exposed. It is rather
niche and hard to understand. It serves as an escape hatch for
rare/extreme cases.

These can still be added with appropriate notes if there is enough
user demand.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>