Add Flipt Java SDK backend

[alexcardell]

This scraps the original API and intention to resolve segments locally. That is something to investigate but for a pragmatic start I'm mirroring the Flipt API and will tinker with some other SDKs and find the common effectful vs pure parts.

Flipt seems to make a HTTP request for everything, including getting flags. It is possible to list multiple flags but it's unclear if the API automatically paginates, meaning we could miss flags trying to get them all up front. It also evaluates segments server side, however the ideal goal here is fetch the whole flag state once in request middleware, cache it, and have enough info to resolve segment allocation locally.

Will eventually tinker with a pure Scala Flipt reimplementation, and see how much can be done app-side instead of server side.