Config server switch to switch between FastCS and Classic Eiger

As FastCS eiger is not yet mature, we should add a switch in the config server feature settings to enable it, the default should be disabled.

## Acceptance Criteria
- The config server has been updated to support such as switch
- Hyperion has been updated to use the switch

Start by locating the config server feature settings and the Hyperion code that consumes them. Add a disabled-by-default switch selecting FastCS or Classic Eiger, then verify that both the config server and Hyperion honor it and that the acceptance criteria are met.

Written by the indexing model from the issue text.