Extract XP side to a library

[espen42]

Should probably contain:

• The frontend proxy
• A service for the meta call: content-path => content-type No. This is resolved with the same guillotine API in the new approach.
• Guillotine endpoint? No. Host app should supply its own, this is independent from the frontend proxy funxtionality.
• A service for listing content-paths, used for Next.js SSG (XP side: API for listing content by paths #47 ). No. This is STILL NEEDED, but probably not as part of a frontend proxy lib repo. Independent from that functionality, maybe a lib of its own? Continue on that issue, closing this one.

[espen42]

@sigdestad - Should the library also contain the actual guillotine API endpoint? If yes, the library is standalone. If no, needs a second library or custom implemention, but is easily pluggable into for instance HMDB, which has one. Opinion?

[sigdestad]

Lets focus on creating the guide and demo, then we'll figure this out later

[espen42]

Needed for the demo, since we're starting with pure HMDB checkout, needs a library to add:

https://github.com/enonic/lib-frontend-proxy