Rework schema initialization for better performance

[bryanlb]

Summary

Reworks the schema initialization for much better performance on load. Internal benchmarks shows this has reduced the cache node schema initialization from ~5s on average for 2k fields down to ~ 15ms for the same amount of fields.

This has the added benefit of preventing field conflicts from occurring on the cache nodes, as you first establish the entire nested schema before attempting to merge it into the mapper service.

This also does related work of moving significant amounts of OpenSearch adapter code into static initialization where possible to further improve performance loading a large amount of chunks. This work was split out of #1052.