[Feature Request] Alias column in schema (virtual column)

It would be nice to have the ability to define in the schema a virtual or alias column which simply causes a different physical column to be fetched at query time. This would mirror the exact same functionality as `SELECT foo AS bar`, except that assigning the alias at query time would not be required. It would be required that any alias column could not share a name with a physical column.

In particular, this feature would allow for the support of a UNION like operation when combined with the notion of table aliasing[1].

[1] https://github.com/apache/pinot/issues/10712

The issue does not name files, tests, or code entry points. Start by reviewing the linked table-aliasing issue and the existing schema and query behavior around SELECT foo AS bar. Done would require an agreed design and implementation for schema-defined virtual columns, including the physical-column name conflict rule and the proposed UNION use case.

Written by the indexing model from the issue text.