net: DHCPv4: Add option callback

In certain scenarios, it may be necessary to get values of additional
options from the application layer. With this patch, this can be
accomplished by registering a callback with the DHCP client.

This change has been tested using the posix build in qemu.

Signed-off-by: Matthias Breithaupt <m.breithaupt@vogl-electronic.com>