Add nightly track for nested vector search (ala semantic_text, no inference)

[jilldoty-elastic]

We need a realistic nested vector index and retrieval nightly track. This task is to build a "semantic text" (esque) benchmark that indexes passage vectors with varying passage counts (1 to 10s to 100s).

The queries should do:

• Semantic text (esque) searches (I think just kNN if we don't want inference in there)
• kNN with passage highlighting
• kNN with inner_hits scoring (I have just been seeing customers do this).

Future / out of scope

• add benchmark with EIS