[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).

---------

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

cumulus/parachains/common/src/lib.rs

@@ -17,7 +17,9 @@
 
 pub mod impls;
 pub mod message_queue;
+#[cfg(feature = "rococo")]
 pub mod rococo;
+#[cfg(feature = "westend")]
 pub mod westend;
 pub mod xcm_config;
 pub use constants::*;