fix: metadata datatype

[marc0olo]

@davidyuk @dincho I get NodeInvocationError for credentialNFTTest.js:

NodeInvocationError: Invocation failed: "bad_call_data"
      at handleCallError (node_modules/@aeternity/aepp-sdk/dist/aepp-sdk.js:8055:11)
      at Object.instance.call (node_modules/@aeternity/aepp-sdk/dist/aepp-sdk.js:8219:7)
      at processTicksAndRejections (node:internal/process/task_queues:96:5)
      at async Object.instance._estimateGas (node_modules/@aeternity/aepp-sdk/dist/aepp-sdk.js:8087:9)
      at async Object.instance.call (node_modules/@aeternity/aepp-sdk/dist/aepp-sdk.js:8232:108)
      at async Context.<anonymous> (test/credentialNFTTest.js:65:19)

I have no idea what the problem is. calldata usage seems to be fine but somehow it seems to be a bug. otherwise the node wouldn't complain 🤷‍♂️

I know it's friday evening. but it would be awesome to have this fixed asap because I need to make a presentation about how to create an NFT 😅

[marc0olo]

actually I am not sure if I did everything right. but I assume the error is somewhere in the calldata-lib producing wrong calldata. I might be wrong

[marc0olo]

example can now be used but long keys in the MetadataMap can currently not be used until aeternity/aepp-calldata-js#160 is sorted out