[cumulus] Feature gates for rococo/westend related stuff in the parachains-common

[bkontur]

This PR avoids automatically pulling rococo/westend runtime constants into the runtime by default. Usually, we have testnet runtimes dedicated to rococo or westend, and therefore, we don't need both dependencies. Additionally, it prevents pulling rococo/westend-related items into the polkadot-fellows repo, as seen in the Cargo.lock here and here.

[acatangiu]

nice!

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable 2/3
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5015374

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable 1/3
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5015373

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable 3/3
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5015375

[bkchr]

The stuff in the crate that is only for the fellowship runtimes, should be put into a crate in the fellowship repo. Fellowship should really only depend on pallets on common structs, nothing that is specialized for any kind of chain.

[bkontur]

The stuff in the crate that is only for the fellowship runtimes, should be put into a crate in the fellowship repo. Fellowship should really only depend on pallets on common structs, nothing that is specialized for any kind of chain.

Actually, I didn't check other stuff inside (just that rococo/westend stuff). Afaik, parachains-common contains just a common specific-chain-agnostic utilities, but I see some stuff which could be cleaned up. I will continue here: #3054