Pluggable Key-Value Store for Upserts

Pinot upserts require an [in-memory map](https://github.com/apache/pinot/blob/bc07eb8e7fa1ea4ef26ebbcd1a2ce2087a3058e4/pinot-segment-local/src/main/java/org/apache/pinot/segment/local/upsert/ConcurrentMapPartitionUpsertMetadataManager.java#L54) that tracks the primary keys to the corresponding record locations. This is used to find if a record with that primary key already exists and if so, merges that with the incoming record. For tables with a large number of primary keys, this leads to huge memory consumption because this map is stored in the heap memory.

In certain use-cases, we came across tables that required a longer retention period and a strict level of correctness, so we explored alternatives on replacing this in-memory map with disk-backed maps. However, the current implementation (see ConcurrentMapPartitionUpsertManager) is heavily coupled with the in-memory map (Java’s ConcurrentHashMap). This reduces the flexibility for Pinot adopters to replace this Map with their own implementation of this map.

Creating this issue to discuss if we can extract an interface out of this to make the "Map" pluggable and to gather community's feedback.

This [write-up](https://docs.google.com/document/d/1jsu9qChX3set560ll3ClGvorqpTP5yhsZn-RjSTE_eY/edit) talks about this idea in detail.

cc @tibrewalpratik17

Start with ConcurrentMapPartitionUpsertMetadataManager.java and the referenced write-up to understand how the in-memory map is coupled to Pinot upserts. The issue seeks community feedback on extracting a pluggable Map interface, so the exact design and completion criteria remain unresolved.

Written by the indexing model from the issue text.