auto installer: factor out fetch-answer and autoinst-helper

auto installer: factor out fetch-answer and autoinst-helper

Putting proxmox-fetch-answer into it's own crate, will keep the use of
OpenSSL localized to where we need it. Otherwise building other binaries
will always depend on OpenSSL as well, even without actually needing it.

Having a dedicated crate for the proxmox-autoinst-helper should make it
easier to build it independently to have it available outside of the
install environment.

The fetch plugins have been moved to the proxmox-fetch-answer crate,
except for the 'get_nic_list' function and 'sysinfo.rs'. Since both are
also needed by the proxmox-autoinst-helper, they are kept in the
proxmox-auto-installer crate.

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>