Summary:
This diff adds a simple test for Vector LSM.
It inserts a batch of vectors via the provided interface and performs search, checking its results.

Since Vector LSM performs actual insert in the background, the test performs search until the insert operation into the underlying vector index completes.
The test checks that the results are correct both while the background insert is running and after the background insert is complete.

Also:
1) Implemented search in active tasks, so it is opaque to the VectorLSM user that the actual vector index insert happens in background.
2) Fixed multiple small issues found during test development.
3) Implemented format for storing and fetching vector id to base table key mapping in RocksDB in test. That should be moved to the right place in code in followup diffs.
Jira: DB-13135

Test Plan: vector_lsm-test

Reviewers: mbautin

Reviewed By: mbautin

Subscribers: slingam, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D38449