List of all routes

https://github.com/ktorio/ktor/issues/788**Subsystem**
Server

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
Things like spring actuator are able to list all the endpoints in the current system. Something like this for ktor would be very helpful.

**Motivation to include to ktor**
A very common usage so makes sense to build directly into ktor.

No files or tests are named. Start in the Ktor Server subsystem and review the linked issue and the Spring Actuator comparison to understand the expected route and endpoint listing. Done means Ktor can expose all endpoints in the current system.

Written by the indexing model from the issue text.