Cyphal/UDP: MTU shall remain constant for the duration of a transfer

Context: https://matrix.to/#/!iIoEluFjwtvXWxhOJg:matrix.org/$TM4A46iRgBFc64yi1hev5Uuam3fAsZGgvMpD-vunkfc?via=matrix.org&via=lenfesty.ca&via=stratum0.org

Also, full MTU utilization should be required for all frames of a transfer except the last one. MTU discovery is required for out-of-order reassembly.