Revisit pallet-xcm-benchmarks after a while.

[ruseinov]

After those benchmarks have been implemented for several parachains we should see whether we need to maintain a duplicate of this polkadot pallet or just backport all of the changes back to polkadot.

In case we decide to maintain a separate code base further - rename the pallet to avoid clashing names as polkadot's bench pallet is an indirect dependency which pops up in Cargo.lock.

Renaming options: cumulus-pallet-xcm-benchmarks, pallet-xcm-parachain-benchmarks or whatever else could be shorter and remain eloquent.

Continuation of:
#1056
#1454