selftests/bpf: test_xdp_veth: Create struct net_configuration

selftests/bpf: test_xdp_veth: Create struct net_configuration

The network configuration is defined by a table of struct
veth_configuration. This isn't convenient if we want to add a network
configuration that isn't linked to a veth pair.

Create a struct net_configuration that holds the veth_configuration
table to ease adding new configuration attributes in upcoming patch.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250212-redirect-multi-v5-1-fd0d39fca6e6@bootlin.com