[feature][CDC] The basic implementation of the CDC source enumerator in the snapshot phase

[ashulin]

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Description

This is a subtask of #3175 to track completion.

Snapshot phase

The enumerator generates multiple SnapshotSplits of a table and assigns them to the reader

//  pseudo-code. 
public class SnapshotSplit implements SourceSplit {
    private final String splitId;
    private final TableId tableId;
    private final SeaTunnelRowType splitKeyType;
    private final Object splitStart;
    private final Object splitEnd;
}

When a SnapshotSplit reading is completed, the reader reports the high watermark of the split to the enumerator,
When all SnapshotSplits report high watermark, the enumerator enters the incremental phase.

//  pseudo-code. 
public class CompletedSnapshotSplitReportEvent implements SourceEvent {
    private final String splitId;
    private final Offset highWatermark;
}

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• Yes I am willing to submit a PR!

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