deprecate file.upload and implement new apis

[koonwen]

Related to #5

I did some initial scaffolding work around this. Haven't done any testing yet so it probably doesn't work.

Some caveats about the new api is that it is a three step query process

1. get a url to upload to
2. actually upload the file to the url
3. specify where the uploaded file should appear in slack

Also with the new api, we cannot specify multiple channels in 3. and you can only call it once which I guess means once you upload it once, it gets thrown away after. https://api.slack.com/methods/files.getUploadURLExternal#markdown I thought about multiplexing the request by getting multiple upload urls but I don't know if this will work as expected and slack will give back multiple unique ids to upload to. That's also kinda inefficient to duplicate the upload work.

In any case, feel free to take over this PR

[sewenthy]

I feel like this change with additional upload_file_v2 is a bit hard to manage, I would rather remove the deprecated version. Honestly, not sure how to handle deprecation in opensource libraries 😅 @Khady if you have any suggestions!