Makes constant endpoint an api with implementations available

[codefromthecrypt]

In a former change @anuraaga rightly mentioned that there is trouble putting concrete types in the HttpSupplier interface file. I noticed spring has its own type they use as a constant supplier, ZipkinConnectionDetails. Since this is an interface, this double-underscores the same concern.

This hopefully clears up the topic, as well making a place to implement a rate-limited supplier, something sleuth has, but boot doesn't yet.

[reta]

Looks pretty clean! 🥇

[codefromthecrypt]

glad you like it, missing rate limiter coming in a minute. This is important as eureka isn't designed to respond fast (at least I notice some latency). We also need it to rate limit when there are errors.