Allow unnested field to be renamed during ingestion of complex type

Pinot supports unnesting a complex type as documented here: https://docs.pinot.apache.org/basics/data-import/complex-type#handle-the-complex-type-with-ingestion-configurations

As part of this operation, nested fields appear as flat columns in Pinot with this naming convention: '.'.

It might be useful to rename such "unnested" fields to a user chosen config. For instance, user may choose to only retain '' in the above example. There may be other ways to specify this renaming policy.

Start with the complex-type ingestion configuration documentation linked in the issue and trace the existing unnesting configuration. Done means a user-configurable renaming policy is defined for flattened fields such as ., with the behavior documented for ingestion users.

Written by the indexing model from the issue text.