[cumulus] Clean `parachains-common`

[bkontur]

Context: Basti's comment Continuation of https://github.com/paritytech/polkadot-sdk/pull/3041

TODO

• [x] create new module cumulus/parachains/runtimes/constants
  • [x] move rococo.rs and westend.rs there (similar as polkadot-fellows has system-parachains-constants module)
• [ ] move mod constants to the new module (fellows has its own copy already)
• [ ] remove AssetHubPolkadotAuraId a move it to the polkadot-fellows
• [ ] revisit mod types and mod opaque

Questions:

• [ ] What about common types like: AccountId, Balance,.... Can we share them between polkadot-sdk and polkadot-fellows repo? Or better split all of them?
  • [ ] What is the purpose of these types here? To be shared/used:
  • [ ] by all parachains?
  • [ ] by testnets?
  • [ ] by fellows runtimes?
  • [ ] or should everybody defines them separately? (types for testnet parachains, types for fellows parachain in fellows repo, ...)
• [ ] What about AssetHubPolkadotAuraId? It is used for some stuff in parachain-polkadot module e.g. here and/or here. Remove from parachains-common and have definitions on two places (somewhere in polkadot-parachain and polkadot-fellows)? Or leave as it is?

[seadanda]

IMO for the AssetHubPolkadotAuraId having a definition in fellows and a definition in polkadot-parachain is the way to go, since it belongs with the fellowship and will never change anyway. Also it's just an alias.