test against a real flagd setup

[justinabrahms]

We should have a pre-release check which will spin up a flagd provider and run some java code to validate that the thing actually works. I don't know if this is quick enough to be part of the PR flow (ideal), but it should at least happen before we release new versions.

[justinabrahms]

cc @toddbaert

[toddbaert]

I will take this one (though likely I won't get to it until next week).

I will implement something like what I researched here, centralizing as much as possible.

[toddbaert]

Resolved with #77