Clean up behavior of config parsing

some inconsistencies were discovered after https://github.com/apache/pinot/pull/7497

For example, configuration values were parsed into Lists and then re-constructed back into String. However the return changes were not exactly desired (e.g. parsed as comma-separated string but put back together as Arrays Stringification). This prompts me to ask a question: what are the proper configuration parsing between String / Arrays and other non-primitive types.

Since our usages in pinot-spi are largely depend on apache common configuration package. we should
- add some test around our extended behavior that might or might not be different from apache common. and
- correct any inconsistent / non-desired behavior if necessary

Start by reviewing the configuration parsing behavior used in pinot-spi and compare it with the Apache Commons Configuration package, using PR 7497 as context. Define expected handling for String, Arrays, and other non-primitive values, then add tests for Pinot-specific behavior and correct any inconsistencies those tests expose.

Written by the indexing model from the issue text.