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.
This dependency was removed in https://github.com/4finance/micro-deps/commit/16e3b35e9569d9218211301c9e23b3d98b6fe515, but without it stub-runner doesn't start up with some exception coming from Grape.