Shrink persisted fork choice data (#7805)

Closes:

- https://github.com/sigp/lighthouse/issues/7760


  - [x] Remove `balances_cache` from `PersistedForkChoiceStore` (~65 MB saving on mainnet)
- [x] Remove `justified_balances` from `PersistedForkChoiceStore` (~16 MB saving on mainnet)
- [x] Remove `balances` from `ProtoArray`/`SszContainer`.
- [x] Implement zstd compression for votes
- [x] Fix bug in justified state usage
- [x] Bump schema version to V28 and implement migration.

beacon_node/beacon_chain/src/metrics.rs

@@ -585,6 +585,18 @@ pub static FORK_CHOICE_WRITE_LOCK_AQUIRE_TIMES: LazyLock<Result<Histogram>> = La
         exponential_buckets(1e-3, 4.0, 7),
     )
 });
+pub static FORK_CHOICE_ENCODE_TIMES: LazyLock<Result<Histogram>> = LazyLock::new(|| {
+    try_create_histogram(
+        "beacon_fork_choice_encode_seconds",
+        "Time taken to SSZ encode the persisted fork choice data",
+    )
+});
+pub static FORK_CHOICE_COMPRESS_TIMES: LazyLock<Result<Histogram>> = LazyLock::new(|| {
+    try_create_histogram(
+        "beacon_fork_choice_compress_seconds",
+        "Time taken to compress the persisted fork choice data",
+    )
+});
 pub static BALANCES_CACHE_HITS: LazyLock<Result<IntCounter>> = LazyLock::new(|| {
     try_create_int_counter(
         "beacon_balances_cache_hits_total",