feat: add unified nft collection standard draft

[fabw222]

unft_standard specifies a reusable Move module that streamlines the creation
and management of NFT collections on Iota. It standardizes the metadata layout,
capabilities for minting/burning, optional supply ceilings, and lifecycle
controls such as pausing, while emitting baseline events for indexing.

[lzpap]

Hey @fabw222 , thank you for the submission! I have a couple questions/observations:

• Would it make sense to allow NFTMetadata to have user-defined keys and values (probably in string format)? It may be too limiting to allow only pre-defined keys.
• If I get it right, minting/burning and enforcing the max supply are defined in the developer defined package T, therefore the standard only provides a unified (opt-in) way of tracking the supply. Same goes for pausability.
• It would be amazing to also see an e2e example of a package T that defines an NFT collection and uses this standard for tracking. Having such an example with tests demonstrating usage would help other developers adopting the standard. Feel free to attach move package code in the iips/IIP-0006/ folder.