Add encoded keyword for the HTTPFileSystem

[mraspaud]

This PR adds the encoded keyword argument to HTTPFileSystem to allow ready-encoded urls to be passed unaltered.

The rationale behind this is that aiohttp, by using yarl, canonizes the urls before using them in .get. This works for most servers, as they are compliant with, among others, RFC3986. However, some servers, in particular the one following the OpenAPI specification, do not comply, hence making a mess for example when sending a url containing : in the path part.
To prevent the canonization of the url, aiohttp recommends here passing encoded=True to the yarl.URL object creation before passing it to aiohttp's get. More details and examples can be provided upon request.

This PR hence adds the possibility to configure the HTTPFileSystem to bypass the canonization of the urls.

[mraspaud]

PS: hints on how to test this would be welcome.

[martindurant]

To test, I suppose you would need the server (fsspec.tests.HTTTestHandler) to simply respond back with the full path that was requested, where this mode is activated by passing a special header keyword.

[martindurant]

Do you expect to be able to make a test like I suggested?

[martindurant]

Please check that the test I added makes sense to you.

[mraspaud]

@martindurant thanks a lot for adding the test! I've been travelling those last weeks, so I didn't have time to look at this before today. But it looks good, I just added a “:“ character as that was the one I was struggling with. So all good for me now!