Add NFTs pallet

[stiiifff]

The FRAME Uniques pallet (currently included in the Trappist runtime) has been deprecated in favor of the newer and more versatile NFTs pallet. As we want to showcase some NFT-related capabilities of Polkadot / Substrate, we should remove the old Uniques pallet and add the NFTs pallet to the Trappist runtime, and schedule a runtime upgrade for Trappist Rococo.

Note: proper storage migration logic should be implemented in the runtime in case there exists some NFT collections on-chain implemented with the old Uniques pallet (cfr. #282).