Add AccessTimeMapConfig Thrift struct for ATM persistence

Summary:
Add a new `AccessTimeMapConfig` Thrift struct to the navy serialization
schema. This struct stores the metadata needed to validate and restore
AccessTimeMap entries during warm restart: version, numShards, maxSize,
and entryCount.

This is a pure schema addition with no consumers yet — the persist/recover
methods that use it will be added in a follow-up diff.

Reviewed By: byahn0996

Differential Revision: D95332902

fbshipit-source-id: e4cf5a536490289b889f309df8d6857d23e955fb

Author: pbhandar2

cachelib/navy/serialization/objects.thrift

@@ -112,3 +112,10 @@ struct FixedSizeIndexConfig {
   3: byte numBucketsPerChunkPower = 0;
   4: i64 numBucketsPerShard = 0;
 }
+
+struct AccessTimeMapConfig {
+  1: i32 version = 0;
+  2: i64 numShards = 0;
+  3: i64 maxSize = 0;
+  4: i64 entryCount = 0;
+}