examples: add map in map example
examples: add map in map example

This example demonstrates the use of maps in maps in pure Go.
There's also use of the BPF_F_INNER_MAP flag which allows for
inner maps to be sized dynamically.

Signed-off-by: Mark Pashmfouroush <f1b5a91d4d6ad523f2610114591c007e75d15084@isovalent.com>

cilium/ebpf,cilium/ebpf