feat: better default stream support in client library

[yirutang]

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:

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Fixes #<issue_number_goes_here> ☕️

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/generated-files-bot.yml

[stephaniewang526]

PTAL below error:

Error:  /home/runner/work/java-bigquerystorage/java-bigquerystorage/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1beta2/BQV2ToBQStorageConverter.java:1: Line does not match expected header line of '^/\*$'. [RegexpHeader]

[stephaniewang526]

One unit test failed:

Error:  Failures: 
Error:    JsonStreamWriterTest.testTwoParamNewBuilder_nullSchema:220 expected:<Stream[OrTable]Name is null.> but was:<Stream[]Name is null.>

[stephaniewang526]

Getting NPE in ITBigQueryWriteManualClientTest.testJsonStreamWriterBatchWriteWithDefaultStream

java.lang.NullPointerException
	at com.google.cloud.bigquery.storage.v1beta2.TableFieldSchema$Builder.setMode(TableFieldSchema.java:1647)
	at com.google.cloud.bigquery.storage.v1beta2.BQV2ToBQStorageConverter.ConvertFieldSchema(BQV2ToBQStorageConverter.java:69)
	at com.google.cloud.bigquery.storage.v1beta2.BQV2ToBQStorageConverter.ConvertTableSchema(BQV2ToBQStorageConverter.java:56)
	at com.google.cloud.bigquery.storage.v1beta2.JsonStreamWriter.newBuilder(JsonStreamWriter.java:253)
	at com.google.cloud.bigquery.storage.v1beta2.it.ITBigQueryWriteManualClientTest.testJsonStreamWriterBatchWriteWithDefaultStream(ITBigQueryWriteManualClientTest.java:320)
...

full trace

[yoshi-code-bot]

Suggested change

	import com.google.cloud.bigquery.StandardSQLTypeName;
	import com.google.cloud.bigquery.storage.v1alpha2.Table;
	import com.google.cloud.bigquery.StandardSQLTypeName;

[codecov]

Codecov Report

Merging #750 (0466702) into master (f3c897f) will increase coverage by 0.08%.
The diff coverage is 82.50%.

@@             Coverage Diff              @@
##             master     #750      +/-   ##
============================================
+ Coverage     80.80%   80.89%   +0.08%     
- Complexity      985      994       +9     
============================================
  Files            73       74       +1     
  Lines          5289     5339      +50     
  Branches        406      414       +8     
============================================
+ Hits           4274     4319      +45     
- Misses          845      848       +3     
- Partials        170      172       +2     

Impacted Files	Coverage Δ	Complexity Δ
...oud/bigquery/storage/v1beta2/JsonStreamWriter.java	78.33% <79.16%> (+1.14%)	14.00 <2.00> (+2.00)
...e/cloud/bigquery/storage/v1beta2/StreamWriter.java	85.01% <82.60%> (-0.08%)	38.00 <2.00> (+1.00)	⬇️
...uery/storage/v1beta2/BQV2ToBQStorageConverter.java	84.84% <84.84%> (ø)	6.00 <6.00> (?)
.../cloud/bigquery/storage/v1alpha2/StreamWriter.java	84.31% <0.00%> (+0.43%)	37.00% <0.00%> (ø%)

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