Improve the error messages from GA4GH config options

Currently if you enable the permissions API but don't provide the GA4GH signing keys, you will get an error from the API call. We can improve by providing a better startup validation and even fail to start on such a combination. We can also check the same for EGA related configuration that requires the same keys.

- [ ] check validity of permission API configs including feature flag and the keys
- [ ] check validity of EGA configs including feature flag, keys and servers

Start by locating the startup configuration handling for the permissions API and EGA settings described in the issue. Check the feature flags, signing keys, and EGA servers together, then verify that invalid combinations fail during startup with clear errors rather than only failing on an API call.

Written by the indexing model from the issue text.