net: lwm2m: firmware: add support for firmware pull over CoAP proxy

CoAP allows a proxy to be used when transferring data (CoAP-CoAP and/or
CoAP-HTTP) by creating request on a specific URI path and by using the
Proxy URI CoAP option. Create specific Kconfig options for the proxy
server address and port, until a parser gets implemented.

Code tested with Californium acting as CoAP proxy.

Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>
[michael.scott@linaro.org: rebased on net_app + lwm2m_message
refactoring + firmware update changes.]
Signed-off-by: Michael Scott <michael.scott@linaro.org>