API scope/endpoint to get a blend url

[kevinlinxc]

Hi, the way to get a new Blend URL right now, from a developer perspective, is to hackily use the non-public Spotify API (https://spclient.wg.spotify.com/blend-invitation/)

I think this would be pretty simple to make this a part of the public API, as a separate scope option.

My use case is that I want it to be easier to make a blend with whoever I'm driving in my car, so I want to make an NFC tag that hits my website, which will be using my Spotify app which has this blend scope.