net: lwm2m: add support for coap2coap proxy

Currently, LwM2M firmware download only supports coap2http proxy.
Let's add support for coap2coap proxy as well.

This was tested running Californium demo app cf-proxy on the host
machine with the following setting changed in Californum.properties:
MAX_RESOURCE_BODY_SIZE=524288

Add the following to the samples/net/lwm2m_client/prj.conf:
CONFIG_LWM2M_FIRMWARE_UPDATE_PULL_COAP_PROXY_SUPPORT=y
CONFIG_LWM2M_FIRMWARE_UPDATE_PULL_COAP_PROXY_ADDR="coap://[2001:db8::2]:5682"

Build the sample for qemu_x86 as you would normally, but now
you can use a real world coap address to pull firmware using the 5/0/1
resource.  The host machine running cf-proxy will pull the remote
resource and then deliver it to the running qemu sample.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>