Support asynchronous upload of images

[Gowee]

Hi!

There are some persistent issues with Wikimedia's servers (T335243, T340901, T340917), which effectively prevents large files to be uploaded.

It appears that the problem can be mitigated by toggling the async flag when calling API:Upload, as noted here. I have conducted tests utilizing the async option of pywikibot, resulting in notable improvements in upload stability when dealing with files of several gigabytes in size.

I suggest that we consider incorporating this option into the mwclient library. This could lead to a more efficient and smoother uploading process for users who handle large files via the library.