New multi-router configuration system based on https://hackmd.io/G1KRDEX5T3qyfoBMkIrBew#Methods

- Added a new routing type: "custom"
- Added specific struct types for different Routers (instead of map[string]interface{})
- Added `Duration` config type, to make easier time string parsing
- Added config documentation.
- Use the latest go-delegated-routing library version with GET support.
- Added changelog notes for this feature.

It:
- closes #9157
- closes #9079
- closes #9186