Summary:
YbHnsw organizes data into blocks, which are loaded on demand. When new search operations require different blocks, previously loaded ones may need to be unloaded.

This diff implements block cache to manage:
- Dynamic loading of required blocks
- Unloading of inactive blocks when new blocks are needed

This ensures efficient memory usage while maintaining fast access to frequently used data.
Jira: DB-16564

Test Plan:
HnswTest.Cache
HnswTest.ConcurrentCache

Reviewers: arybochkin

Reviewed By: arybochkin

Subscribers: ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D43752