File data sources: support files in IPFS directories

[azf20]

IPFS file data sources based on files in directories (e.g. https://ipfs.io/ipfs/QmaXzZhcYnsisuue5WRdQDH6FDvqkLQX1NckLqBYeYYEfm/1.json) fail with the following error:

        "fatalError": {
          "message": "Failed to parse multihash",
        }

Example mapping:

token.tokenURI = "/" + event.params.tokenId.toString() + ".json";
token.ipfsURI = ipfshash + token.tokenURI;
TokenMetadataTemplate.create(token.ipfsURI);

File data sources should be able to reference these kind of files as well as a CID