feat: well-defined storage format

[erhant]

Right now we simply check the input string, and if its 64 character hexadecimal string we treat it as an Arweave txid, doing some transformations etc.

It should be easier parse a JSON string like {type: String, key: String} instead, and if type == "Arweave" then simply use arweave.net/{key} to download it; the key is therefore a base64url encoded string in this case. This opens up the way for alternative storage providers as well, (Codex / Walrus)