[cumulus] Feature gates for rococo/westend related stuff in the `parachains-common` (#3041)

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](https://github.com/polkadot-fellows/runtimes/blob/main/Cargo.lock#L14137-L14151)
and
[here](https://github.com/polkadot-fellows/runtimes/blob/main/Cargo.lock#L9756-L9770).

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Co-authored-by: Adrian Catangiu <adrian@parity.io>