Single text index per segment

We recently observed use cases with 100+ columns requiring text indices, which causes the issue of too many open files as current text index implementation is per column.

The proposal here is to allow one shared Lucene Index for Multiple Columns in Pinot.

## Changes
- One Lucene index directory per segment, not per column.
- Each text column is represented as a Lucene field within that shared index.
- TextMatch queries can specify which column (i.e., Lucene field) to search.

## Challenges
- Meanwhile the down side is also obvious, that whenever there are changes required for adding/removing column index, the process may take longer time for rebuilding the whole index.
- How to handle null/missing value
- shared index dir is a fixed path per segment

# #Future extentions
- Advanced queries: Support cross-field Lucene queries if needed later.

No files or tests are named. Start by tracing Pinot's current per-column Lucene text-index implementation and how TextMatch queries select a column. Define completion as one Lucene index directory per segment, fields for each text column, supported null or missing values, and working column-specific TextMatch queries.

Written by the indexing model from the issue text.