webhdfs: migrate to fsspec

[isidentical]

To complete the fsspec migration, we need to migrate webhdfs to the fsspec.

[isidentical]

The fsspec.implementations.webhdfs.WebHDFS file system is hand-written (using the raw HTTP API), and does not depend on the same interface as us (hdfscli).

Because of this, we need to manually parse the hdfscli.cfg's before delegating the information to the fsspec's WebHDFS. One issue I see is kind of relevant is that, since we won't be using *Clients from the hdfscli any more, we can't support some advanced features of the configs like this: https://hdfscli.readthedocs.io/en/latest/advanced.html#custom-client-support

I would assume they are pretty rare, or even unused at all but I don't have much context on this (CC: @efiop).

[efiop]

@isidentical Hm, hdfscli.cfg is a hdfscli-specific config right? I actually don't think that we've documented it ever (need to check though). If that's the case, we could just drop it.

[isidentical]

It is documented

[efiop]

@isidentical Oh 🙁 Well, still, as you've noted, this is pretty rare and hdfscli-specific, so we can consider dropping it since we are switching the backend.