Define CustomNetworkId

Right now we use some associations between Rialto, RialtoParachain and
Millau chains and chains defined in the NetworkId enum. But if we are
not carreful we might do mistakes like:
In Millau:
pub const ThisNetwork: NetworkId = Kusama;
pub const RialtoNetwork: NetworkId = Polkadot;
In Rialto:
pub const ThisNetwork: NetworkId = Kusama;
pub const MillauNetwork: NetworkId = Polkadot;

We're introducing CustomNetworkId to have a centralized mapping between
NetworkId chains and our custom chains.

Signed-off-by: Serban Iorga <[email protected]>