Vector embeddings support in Pinot

Creating this issue to initiate discussions about supporting vector embeddings in Pinot.

This [write-up](https://docs.google.com/document/d/1aiXPbwK4rU_YdfMPt3K752SuCMy8KQehqM4ltPg9juE/edit) collates some initial thoughts about this. It isn't a design doc, we'll work on the design doc once we've a high-level alignment.

Start by reading the issue and the linked write-up to understand the initial thoughts on vector embeddings in Pinot. The issue says the write-up is not yet a design document, so a high-level design and alignment would need to come before implementation; no files, tests, or concrete completion criteria are named.

Written by the indexing model from the issue text.