refactor: use explicit client keyword arguments

[sisp]

I've refactored client keyword arguments to use explicit ones instead of mixing those with **kwargs, which has a few advantages:

1. It's more explicit.
2. With more comprehensive typing – perhaps a follow-up PR 😉 –, typing those arguments would be easier when they are explicit.
3. AFAIK, the previous timeout argument in HTTPFileSystem.__init__() was a dummy because the HTTP(S) config schemas don't declare a timeout setting, neither via REMOTE_COMMON nor via HTTP_COMMON.
4. Popping those settings in HTTPFileSystem.get_client(), introduced in fix get_client #32, is dirty because it's a side-effect whose "correctness" may depend on order of execution. With the change in this PR, it works without a side-effect.

[efiop]

Thanks @sisp !