Added NetworkId::PolkadotBulletin variant (#2517)

We're going to bridge Polkadot Bridge Hub with [Polkadot Bulletin
chain](https://github.com/zdave-parity/polkadot-bulletin-chain) soon
(and Rococo Bridge Hub with 1:1 copy of Polkadot Bulletin chain even
sooner), so we need a variant for that chain in `NetworkId`. As
suggested, I'm adding a new variant for it to the `NetworkId` (we may
have used `ByGenesis(_)`, but decision was made to have a dedicated
variant for that).