net: lwm2m: Allow CoAP block size to be changed

Allow changing the CoAP Block-wise transfers block-size
for subsequent GET requests.

It looks like Leshan switches block size back to its
configured value, if it is smaller.
So even when we send block N=0 with size of 512, Leshan
seem to handle that properly but still asks N=2 with
block size 256(if that is configured).

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>