API add support for libraries and follow

Hello,

I have some friends who are trying to fetch public librairies uuid to auto-follow them (as in #1010 (closed) ). But they lack some api end-point to do it :

• Ability to ask a list of all public libraries in a pod.
• Ability to follow a library

For now we have to extract libraries uuid from the albums. This lead to a lot of requests.

thx :)