auto-installer: fetch: add http plugin to fetch answer

auto-installer: fetch: add http plugin to fetch answer

This plugin will send a HTTP POST request with identifying sysinfo to
fetch an answer file. The provided sysinfo can be used to identify the
system and generate a matching answer file on demand.

The URL to send the request to, can be defined in two ways. Via a custom
DHCP option or a TXT record on a predefined subdomain, relative to the
search domain received via DHCP.

Additionally it is possible to specify a SHA256 SSL fingerprint. This
can be useful if a self-signed certificate is used or the URL is using
an IP address instead of an FQDN. Even with a trusted cert, it can be
used to pin this specific certificate.

The certificate fingerprint can either be placed on the `proxmoxinst`
partition and needs to be called `cert_fingerprint.txt`, or it can be
provided in a second custom DHCP option or a TXT record.
If no fingerprint is provided, we switch rustls to native-certs and
native-tls.

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>