DatastoreV1.Write, DeleteEntity and DeleteKey can optionally return PCollection

[bitnahian]

Addresses #20219

• Introduces withResults() API to DatastoreV1.Write, DatastoreV1.DeleteEntity and DatastoreV1.DeleteKey PTransforms, which returns PCollection<DatastoreV1.WriteSuccessSummary> instead of PDone.
• Allows Wait.on semantics for DatastoreIO, e.g.

    PCollection<Entity> firstBatch = ... ;
    PCollection<Entity> secondBatch = ... ;
    PCollection<DatastoreV1.WriteSuccessSummary> firstWriteResults =
        firstBatch.apply(DatastoreIO.v1().write().withProjectId(project).withResults());

    secondBatch
        .apply(Wait.on(firstWriteResults)) // Allows Wait.on
        .setCoder(secondBatch.getCoder())
        .apply(DatastoreIO.v1().write().withProjectId(project)); // Returns PDone without withResults()

• Maintains backward compatibility with existing API structure
• Draws inspiration from JdbcIO and FirestoreIO connectors
• Modifies unit tests to mock datastore stub to return CommitResponses
• Introduces new integration tests to test the withResults() API

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