Primarily, this PR splits out the NFT contract message structs into their own package called collectxyz for easier integration into other contracts that wish to call the xyz NFT contract. Additionally, it includes a couple small fixes and enhancements, which live in their own commits for easy review.

This PR updates the NFT contract's nft_info and all_nft_info query responses to adhere to OpenSea metadata standards. These changes should make our NFT contract behave like a cw721 v0.9.2 contract, even though we still need to use v0.9.1 under the hood because of how our tokens are represented in the contract storage (at least for now).

Without these updates, nft_info returns token metadata that looks like:

{
  "name": "xyz #1",
  "description": "Explore the metaverse, starting with xyz.",
  "image": "data:image/svg+xml;base64,...",
  "extension": {
    "coordinates": {
      "x": 0,
      "y": 0,
      "z": 0
    },
    "prev_coordinates": null,
    "arrival": "123456..."
  }
}

With these updates, nft_info returns token metadata that looks like:

{
  "token_uri": null,
  "extension": {
    "name": "xyz #1",
    "description": "xyz is a...",
    "attributes": [
      {
        "display_type": null,
        "trait_type": "x",
        "value": "0"
      },
      {
        "display_type": null,
        "trait_type": "y",
        "value": "0"
      },
      {
        "display_type": null,
        "trait_type": "z",
        "value": "0"
      }
    ],
    "image": "data:image/svg+xml;base64,...",
    "image_data": null,
    "external_url": null,
    "background_color": null,
    "animation_url": null,
    "youtube_url": null
  }
}

Note: this doesn't change the response format for any non-CW721 query methods, like xyz_nft_info.

This PR updates MigrateMsg to contain no data, and removes code from the migration handler that would update the contract config (was in place to simplify fast iteration during initial contract development).

The code on this branch has been built and deployed to the testnet. If we find no issues in manual testing against the testnet contract, we should feel secure migrating the mainnet contract to v0.1.1.

This PR introduces two GitHub Actions workflows pulled from the Mirror Protocol contracts repo - one for linting and testing code, and another for packaging code for GitHub releases.