Log pending compaction bytes in a couple places

This PR adds estimated pending compaction bytes in two places:

- The "Level summary", which is printed to the info LOG after every flush or compaction
- The "rocksdb.cfstats" property, which is printed to the info LOG periodically according to `stats_dump_period_sec`

Test Plan:

Ran `./db_bench -benchmarks=filluniquerandom -stats_dump_period_sec=1 -statistics=true -write_buffer_size=524288`
and looked at the LOG.

```
** Compaction Stats [default] **
...
Estimated pending compaction bytes: 12117691
...
2024/01/22-13:15:12.283563 1572872 (Original Log Time 2024/01/22-13:15:12.283540) [/db_impl/db_impl_compaction_flush.cc:371] [default] Level summary: files[10 1 0 0 0 0 0] max score 0.50, estimated pending compaction bytes 12359137
```

db/internal_stats.cc

@@ -2067,6 +2067,12 @@ void InternalStats::DumpCFStatsNoFileHistogram(bool is_periodic,
       interval_compact_bytes_read / kMB / std::max(interval_seconds_up, 0.001),
       interval_compact_micros / kMicrosInSec);
   value->append(buf);
+
+  snprintf(buf, sizeof(buf),
+           "Estimated pending compaction bytes: %" PRIu64 "\n",
+           vstorage->estimated_compaction_needed_bytes());
+  value->append(buf);
+
   if (is_periodic) {
     cf_stats_snapshot_.compact_bytes_write = compact_bytes_write;
     cf_stats_snapshot_.compact_bytes_read = compact_bytes_read;

db/version_set.cc

@@ -4546,8 +4546,10 @@ const char* VersionStorageInfo::LevelSummary(
     // overwrite the last space
     --len;
   }
-  len += snprintf(scratch->buffer + len, sizeof(scratch->buffer) - len,
-                  "] max score %.2f", compaction_score_[0]);
+  len +=
+      snprintf(scratch->buffer + len, sizeof(scratch->buffer) - len,
+               "] max score %.2f, estimated pending compaction bytes %" PRIu64,
+               compaction_score_[0], estimated_compaction_needed_bytes_);
 
   if (!files_marked_for_compaction_.empty()) {
     snprintf(scratch->buffer + len, sizeof(scratch->buffer) - len,