Reimplementation of Disk Usage API

### Description

There is an opportunity to improve functionality and performance of existing Disk Usage API, through a re-implementation.

Currently, the best tool we have for this is based on a custom Codec that separates storage by field; to get the statistics we read an existing index and write it out using `AddIndexes` and force-merging, using the custom codec. This is time-consuming and inefficient and tends not to get done.

What we could do is estimate the storage of each field by iterating its structures (i.e., inverted index, doc-values, stored fields, etc.) and tracking the number of read-bytes. Since we will enumerate the index, it wouldn't require us to force-merge all the data through `addIndexes`, and at the same time it doesn't invade the codec apis.

Start by locating the existing Disk Usage API, its custom Codec path, and the use of AddIndexes and force-merging. The intended result is a reimplementation that estimates per-field storage by iterating index structures and tracking read-bytes without requiring force-merging or changes to codec APIs.

Written by the indexing model from the issue text.