Support complete service registry introspection

Extra methods to allow full introspection of service registry contents (all services and all instances). `ServiceResolve`r heavily refactored to be type-safe: `ServiceAlias` and `ServicePath`, `URI`.

These changes are required to implement 4finance/micro-infra-spring#79 . Once merged and release, https://github.com/4finance/micro-deps-spring-config/commit/d7bf1e00a45290ad5f3169b1be04886f3c5d1d18 can be pull requested.
Imo we should start using collaborators but in  microservice.json we have
dependencies.

Now we could add collaborators to microservice but we would still need to
leave dependencies for backwards compatibility.

Imo either we do that (and that's the best solution) or we don't touch
microservice.json and use dependencies everywhere.

WDYT?