Fixes a crash when loading the beacon chain config if new fields like
`BLOB_SCHEDULE: []` are present.
Previously, the config loader assumed all values were strings, causing
errors such as:
```
Fatal: Could not load beacon chain config '/network-configs/config.yaml': failed to parse beacon chain config file: yaml: unmarshal errors:
line 242: cannot unmarshal !!seq into string
```
This PR updates the parsing logic to handle non-string values correctly
and adds explicit validation for fork fields.
