Take into account key size for MemorySizedCache

[ncoiffier-celonis]

Description

Attempt to fix #6719.

• create a MemUsage trait
• implement MemUsage for CacheKey and SliceAddress
• modify the 3 base cache implementations to take into account the key mem_usage when inserting and reporting the cache in_cache_num_bytes and evict_num_bytes
• hits_num_bytes still return the value size only, without taking into account the key size.
• the changes should be mostly self-contained in the quickwit-storage/cache module

How was this PR tested?

Unit tests only.