support connector-developer-defined statsitem by adding add interfaces in ConnectStatsManager

adding initAdditionalItems(List additionalItems);incAdditionalItem(String additionalItem, String key, int incValue, int incTimes);removeAdditionalItem(String additionalItem, String key) three methods in ConnectorStatsManager to support connector developers record user-defined metrics.
StatsItemSet support delValue in version 4.7.1, so upgrade rocketmq.version to 4.7.1

ConnectStatsManager.java
```java
public void initAdditionalItems(List additionalItems) {
for (String additionalItem : additionalItems) {
if (this.statsTable.containsKey(additionalItem)) {
log.warn("Already exists statsItem : " + additionalItem + ", just skip");
continue;
}
this.statsTable.put(additionalItem, new StatsItemSet(additionalItem, scheduledExecutorService, log));
}
}

public void incAdditionalItem(String additionalItem, String key, int incValue, int incTimes) {
StatsItemSet statsItemSet = this.statsTable.get(additionalItem);
if (statsItemSet != null) {
statsItemSet.addValue(key, incValue, incTimes);
}
}

public void removeAdditionalItem(String additionalItem, String key) {
StatsItemSet statsItemSet = this.statsTable.get(additionalItem);
if (statsItemSet != null) {
statsItemSet.delValue(key);
}
}
```

Start with ConnectStatsManager.java and inspect how its existing StatsItemSet entries are initialized and updated. Check the StatsItemSet API available after upgrading rocketmq.version to 4.7.1, then verify that the three additional-item interfaces support initialization, increments, and removal as described.

Written by the indexing model from the issue text.