Enable cross-chain NFT transfers on asset-hub (#2796)

This PR introduces the following changes:

- [x] Adds a `UniquesTransactor` to asset-hub-rococo
- [x] Adds a `UniquesTransactor` to asset-hub-westend

We can't add a transactor for `pallet-nfts` like we do for
`pallet-uniques` because `pallet-nfts` uses `nonfungibles_v2::Mutate`
instead of `nonfungibles::Mutate`, and making that work would be out of
scope of this PR.

With these modifications, reserve-based NFT cross-chain transfers can be
performed on asset-hub.

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Co-authored-by: joe petrowski <[email protected]>