feat: generate + mass create NFTs with `pallet-uniques`

[peterwht]

Prerequisite: https://github.com/paritytech/trappist/issues/297 Solves: https://github.com/paritytech/trappist/issues/282 Follow up: https://github.com/paritytech/trappist/issues/283 & https://github.com/paritytech/trappist/issues/285

Provides scripts + building blocks to:

• Bulk create mock NFTs on a local network, no images required.
• Generate NFT images from trait .pngs
• And an e2e example of generating NFTs + metadata from traits, storing them in IPFS (in progress), and creating them on a parachain

Currently, this does not mint NFTs to accounts. Rather, it pre-populates all NFTs with metadata.