Affix settings should work with validators

[stu-elastic]

If you use a validator that returns AffixSettings from Setting.Validator.settings() then trying to perform validation fails with

Caused by: java.lang.UnsupportedOperationException: affix settings can't return values use #getConcreteSetting to obtain a concrete setting

Affix settings should be usable from Setting.Validator.settings()

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Settings)

[stu-elastic]

After addressing this issue:
1) Affix settings will be able to use validators (already done)
2) Affix settings will be usable as dependencies in validators