Add 'autofill_fields' class attribute to 'TomlConfig'.

- Until now, 'TomlConfig' fields were either mandatory of had a
  fixed default value. In some cases, the latter may be refined
  based on other values; however, they could not be missing in
  TOML files.
- With the added 'autofield_fields' class attribute, some fields
  can be explicitly marked as having a dynamically-created default
  value. In that case, it can safely be ignored in TOML files.
- This is now used to support omitting 'evaluate' and 'fairness'
  fields when writing down a 'FLRunConfig' as a TOML file.