Implement write_to_disk for uri backends

[njgheorghita]

What was wrong?

Writing resolved ipfs uris to disk is something I find myself doing a lot manually, and would be useful if available on uri backends.

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[njgheorghita]

Yeah, that's a fair point. I'm not against moving this logic to a util function. I was just thinking it might be cleaner/simpler since you already have the IPFS backend to do something like

ipfs_backend.write_to_disk(uri, target_path)

rather than

import write_uri_to_disk
write_uri_to_disk(ipfs_backend, uri, target_path)

In terms of duplicating the logic, I just moved the definition of write_to_disk to the BaseURIBackend, but if you think this is still better off as a utility function, I can see that point of view.

[njgheorghita]

I guess this relates to how we interact with the ipfs backend in general. Typically, I find myself instantiating a specific backend (i.e. InfuraIPFSBackend / LocalIPFSBackend) and using them directly. But with the current update to resolve_uri_contents(uri) -> contents which asynchronously automatically tries all the backends to fetch the contents, it might make sense to have a similar function write_uri_to_disk(uri, target_path) which is backend agnostic and just tries to fetch from whatever backend is able/available. Encouraging users to skip instantiating their own URI backend to fetch contents and use the utils resolve_uri_contents() / write_uri_to_disk.