feat: support append() without offset in StreamWriter (#1452)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/java-bigquerystorage/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

Author: jphalip

google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1/JsonStreamWriter.java

@@ -80,7 +80,8 @@ private JsonStreamWriter(Builder builder)
 
   /**
    * Writes a JSONArray that contains JSONObjects to the BigQuery table by first converting the JSON
-   * data to protobuf messages, then using StreamWriter's append() to write the data.
+   * data to protobuf messages, then using StreamWriter's append() to write the data at current end
+   * of stream.
    *
    * @param jsonArr The JSON array that contains JSONObjects to be written
    * @return ApiFuture<AppendRowsResponse> returns an AppendRowsResponse message wrapped in an
@@ -92,7 +93,8 @@ public ApiFuture<AppendRowsResponse> append(JSONArray jsonArr) {
 
   /**
    * Writes a JSONArray that contains JSONObjects to the BigQuery table by first converting the JSON
-   * data to protobuf messages, then using StreamWriter's append() to write the data.
+   * data to protobuf messages, then using StreamWriter's append() to write the data at the
+   * specified offset.
    *
    * @param jsonArr The JSON array that contains JSONObjects to be written
    * @param offset Offset for deduplication

google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1/StreamWriter.java

@@ -224,14 +224,23 @@ public void run(Throwable finalStatus) {
   }
 
   /**
-   * Schedules the writing of a message.
+   * Schedules the writing of rows at the end of current stream.
    *
-   * <p>Example of writing a message.
+   * @param rows the rows in serialized format to write to BigQuery.
+   * @return the append response wrapped in a future.
+   */
+  public ApiFuture<AppendRowsResponse> append(ProtoRows rows) {
+    return append(rows, -1);
+  }
+
+  /**
+   * Schedules the writing of rows at given offset.
+   *
+   * <p>Example of writing rows with specific offset.
    *
    * <pre>{@code
-   * AppendRowsRequest message;
-   * ApiFuture<AppendRowsResponse> messageIdFuture = writer.append(message);
-   * ApiFutures.addCallback(messageIdFuture, new ApiFutureCallback<AppendRowsResponse>() {
+   * ApiFuture<AppendRowsResponse> future = writer.append(rows, 0);
+   * ApiFutures.addCallback(future, new ApiFutureCallback<AppendRowsResponse>() {
    *   public void onSuccess(AppendRowsResponse response) {
    *     if (!response.hasError()) {
    *       System.out.println("written with offset: " + response.getAppendResult().getOffset());
@@ -247,7 +256,7 @@ public void run(Throwable finalStatus) {
    * }</pre>
    *
    * @param rows the rows in serialized format to write to BigQuery.
-   * @param offset the offset of the first row.
+   * @param offset the offset of the first row. Provide -1 to write at the current end of stream.
    * @return the append response wrapped in a future.
    */
   public ApiFuture<AppendRowsResponse> append(ProtoRows rows, long offset) {

google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1/StreamWriterTest.java

@@ -131,7 +131,7 @@ private AppendRowsResponse createAppendResponseWithError(Status.Code code, Strin
   }
 
   private ApiFuture<AppendRowsResponse> sendTestMessage(StreamWriter writer, String[] messages) {
-    return writer.append(createProtoRows(messages), -1);
+    return writer.append(createProtoRows(messages));
   }
 
   private ApiFuture<AppendRowsResponse> sendTestMessage(