This PR adds [Rococo
People](https://github.com/paritytech/polkadot-sdk/pull/2281) <> [Rococo
Bulletin](https://github.com/zdave-parity/polkadot-bulletin-chain) to
the Rococo Bridge Hub code. There's a couple of things left to do here:
- [x] add remaining tests - it'd need some refactoring in the
`bridge-hub-test-utils` - will do in a separate PR;
- [x] actually run benchmarks for new messaging pallet (do we have bot
nowadays?).

The reason why I'm opening it before this ^^^ is ready, is that I'd like
to hear others opinion on how to deal with hacks with that bridge.
Initially I was assuming that Rococo Bulletin will be the 1:1 copy of
the Polkadot Bulletin (to avoid maintaining multiple
runtimes/releases/...), so you can see many `PolkadotBulletin` mentions
in this PR, even though we are going to bridge with the parallel chain
(`RococoBulletin`). That's because e.g. pallet names from
`construct_runtime` are affecting runtime storage keys and bridges are
using runtime storage proofs => it is important to use names that the
Bulletin chain expects.

But in the end, this hack won't work - we can't use Polkadot Bulletin
runtime to bridge with Rococo Bridge Hub, because Polkadot Bulletin
expects Polkadot Bridge hub to use `1002` parachain id and Rococo Bridge
Hub seats on the `1013`. This also affects storage keys using in
bridging, so I had to add the [`rococo`
feature](https://github.com/svyatonik/polkadot-bulletin-chain/blob/add-bridge-pallets/runtime/Cargo.toml#L198)
to the Bulletin chain. So now we can actually alter its runtime and
adapt it for Rococo.

So the question here is - what's better for us here
- to leave everything as is (seems hacky and non-trivial);
- change Bulletin chain runtime when `rococo` feature is used - e.g. use
proper names there (`WithPolkadotGrandpa` -> `WithRococoGrandpa`, ...)
- add another set of pallets to the Bulletin chain runtime to bridge
with Rococo and never use them in production. Similar to hack that we
had in Rococo/Wococo

cc @acatangiu @bkontur @serban300 

also cc @joepetrowski

 as the main "client" of this bridge

---

A couple words on how this bridge is different from the Rococo <>
Westend bridge:
- it is a bridge with a chain that uses GRANDPA finality, not the
parachain finality (hence the tests needs to be changed);
- it is a fee-free bridge. So
`AllowExplicitUnpaidExecutionFrom<Equals<SiblingPeople>>` + we are not
paying any rewards to relayers (apart from compensating transaction
costs).

---------

Signed-off-by: dependabot[bot] <[email protected]>
Signed-off-by: Andrei Sandu <[email protected]>
Co-authored-by: Adrian Catangiu <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Andrei Sandu <[email protected]>
Co-authored-by: Egor_P <[email protected]>
Co-authored-by: command-bot <>