auto-installer: add fetch answer binary

auto-installer: add fetch answer binary

it is supposed to be run first and fetch an answer file.

The initial implementation searches for a partition/filesystem called
'proxmoxinst' or 'PROXMOXINST' with an 'answer.toml' file in the root
directory.

Once it has an answer file, it will call the 'proxmox-auto-installer'
and pipe in the contents via stdin.

Tested-by: Christoph Heiss <c.heiss@proxmox.com>
Reviewed-by: Christoph Heiss <c.heiss@proxmox.com>
Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>