Implement image endpoints for playlists

[marioortizmanero]

Is your feature request related to a problem? Please describe.

We're missing the endpoints to read and set a playlist's cover image:

• https://developer.spotify.com/documentation/web-api/reference/#/operations/get-playlist-cover
• https://developer.spotify.com/documentation/web-api/reference/#/operations/upload-custom-playlist-cover

Describe the solution you'd like

Implement these endpoints and update the model.

Additional context

This was originally brought up in #16.

[herberthudson]

Hi,
for the upload I still couldn't test it, I just wrote the code, and since I'm still new to rust, I couldn't think of how I could write tests to validate the code. If you prefer, I can send it in a separate branch if you want to review the code. In any case, when I have more time, I'll come back to it.

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