Make a CIP-30 extension CIP, to allow the "Network magic" to be reported from a wallet.

[stevenj]

We need a way to tell what network a wallet is conencted to, not just if its a tesnet or mainnet.

Cip30: https://github.com/cardano-foundation/CIPs/blob/master/CIP-0030/README.md

CIP30 defines a way to create extensions: https://github.com/cardano-foundation/CIPs/blob/master/CIP-0030/README.md#extension
It also defines a method to return the network: https://github.com/cardano-foundation/CIPs/blob/master/CIP-0030/README.md#apigetnetworkid-promisenumber
However, this only says if the network is mainnet, or not. Its not useful, pre-prod, pre-view or any other test network are all lumped together as 0. We need to be able to tell if a wallet is used Preprod or preview or some other test network reliably.
Actual wallet addresses do not do this, as they could be the same across test networks.

So, create a CIP 30 extension CIP that adds a api.getNetworkMagic(): Promise<number> function which returns the connected networks magic number, so the dapp can determine exactly what test network it is connected to.

for pre-existing examples of cip-30 extensions, See:
https://github.com/cardano-foundation/CIPs/blob/d08e2671148a4f970ad9dab6ea82c9e94ddf32af/CIP-0106/README.md?plain
https://github.com/cardano-foundation/CIPs/blob/d08e2671148a4f970ad9dab6ea82c9e94ddf32af/CIP-0095/README.md?plain
https://github.com/cardano-foundation/CIPs/blob/d08e2671148a4f970ad9dab6ea82c9e94ddf32af/CIP-0104/README.md?plain

The extension should be small and introduce this one feature.