[DISCUSS]Add an internal class to `TransactionBufferStats` to record the snapshot status uniformly.

[Xiangying Meng]

Hi, Community,
We plan to add an internal class to `TransactionBufferStats` to record the
snapshot status uniformly.
As we all know, the current transaction buffer(TB) filters the messages
sent using the aborted transaction by storing the aborted ID in TB.
Then TB will periodically store these aborted txn IDs in a bookie entry in
the form of snapshots so that TB can recover faster when recovering.
But as more and more people use transactions, we found that in some extreme
cases, a bookie entry may not be able to store all aborted transaction IDs.
So in PIP196 <https://github.com/apache/pulsar/issues/16913>, we
implemented the multiple-snapshot function.
As the transaction buffer snapshot mechanism becomes increasingly complex,
the only information related to the transaction snapshot is
`lastSnapshotTimestamps`; That is not enough, we need to add more info to
record the snapshot stats.
So I suggest adding an internal class SnapshotStats to
TransactionBufferStats to record the snapshot status uniformly.

The modification could be :
```java
public class TransactionBufferStats {
    ...
    public long lastSnapshotTimestamps;
    ...
}
```
```java
public class TransactionBufferStats {
    ...
    //public long lastSnapshotTimestamps;
    ...
        public SnapshotStats snapshotStats;

        public static class SnapshotStats {
        public long segmentsSize;

        public long unsealedAbortTxnIDs;


        public long lastSnapshotTimestamps;
    }
}

```
Thanks.
Xiangying