Add rel_size_replica_cache (#11889)

## Problem

See 
Discussion:
https://neondb.slack.com/archives/C033RQ5SPDH/p1746645666075799
Issue: https://github.com/neondatabase/cloud/issues/28609

Relation size cache is not correctly updated at PS in case of replicas.

## Summary of changes

1. Have two caches for relation size in timeline:
`rel_size_primary_cache` and `rel_size_replica_cache`.
2. `rel_size_primary_cache` is actually what we have now. The only
difference is that it is not updated in `get_rel_size`, only by WAL
ingestion
3. `rel_size_replica_cache` has limited size (LruCache) and it's key is
`(Lsn,RelTag)` . It is updated in `get_rel_size`. Only strict LSN
matches are accepted as cache hit.

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

control_plane/src/pageserver.rs

@@ -546,6 +546,11 @@ impl PageServerNode {
                 .map(serde_json::from_str)
                 .transpose()
                 .context("Falied to parse 'sampling_ratio'")?,
+            relsize_snapshot_cache_capacity: settings
+                .remove("relsize snapshot cache capacity")
+                .map(|x| x.parse::<usize>())
+                .transpose()
+                .context("Falied to parse 'relsize_snapshot_cache_capacity' as integer")?,
         };
         if !settings.is_empty() {
             bail!("Unrecognized tenant settings: {settings:?}")