Allow dynamic routing

Add support for modifying routes at runtime, possibly in a separate feature so default behaviour is immutable.

possible use cases:
- runtime modifiable modular system in which the plugins provide the routes.
- feature switch, enable/disable routes dynamically

Start by locating Ktor's routing entry points and related tests; the issue does not name specific files or a failing case. Determine how routes are currently registered and what a separate, optionally mutable feature would need to support for plugin-provided routes and runtime feature switches. Done means the runtime behavior and its interaction with immutable default routing are defined and covered by tests.

Written by the indexing model from the issue text.