Add the concept of a "readiness" similar to k8s, etc.

[alecthomas]

@mistermoe and I chatted about this

Could be something like this:

//ftl:export
func Readiness(ctx context.Context) error {
	_, err := bearerDID.Get(ctx)
	return err
}

[deniseli]

Leaving some quick questions here for whoever ends up picking this up:

1. Should we require the function to be named Readiness or should we rely on an //ftl:readiness directive instead? If so, do we want Readiness to be a reserved name? i.e. no other functions can be named that.
2. Do we want to perform the readiness check (i.e. call each exported Readiness func) in Service.Status() in controller.go?
3. Should readiness effectively be a specialty verb? i.e. if a function is exported with the readiness directive, and only that directive, then it also automatically gets treated as a verb in all the ways a verb would be; it's just also invoked for the readiness checks at startup? It's a bit odd because by the time the module is live, any Readiness function would presumably be returning nil persistently, so there wouldn't really be a point to calling it.
   1. Related: is there any reason to make these exportable? For module-to-module calls, it seems like exporting is just not useful. For calling it from the controller, we could argue that's a public/external access, in which case the visibility should technically be public, but that's basically implied already by the nature of readiness.
   2. tl;dr: I'm leaning towards: Readiness is not a verb. It needs directive //ftl:readiness precisely. No explicit export in the directive. No module-to-module calls. The controller will call this function at startup.

[alecthomas]

I agree re. not being exported, I think //ftl:export in the example predated visibility controls.

I think it should be a verb for consistency though, as that is our unit of work for everything in FTL. I'm fine with either a well known name or //ftl:readiness, though the latter is more explicit.

[deniseli]

Sounds good! I'll pick this up after I finish my other issues if no one else beats me to it (but everyone definitely please feel free to take this!)