feat: add v1beta2, v1alpha2 clients

[chingor13]

Fixes #42
Fixes #43

[codecov]

Codecov Report

Merging #44 into master will decrease coverage by 3.25%.
The diff coverage is 68.43%.

@@             Coverage Diff              @@
##             master      #44      +/-   ##
============================================
- Coverage     74.87%   71.62%   -3.26%     
- Complexity       97      164      +67     
============================================
  Files            11       23      +12     
  Lines           589     1191     +602     
  Branches          1        3       +2     
============================================
+ Hits            441      853     +412     
- Misses          147      335     +188     
- Partials          1        3       +2

Impacted Files	Coverage Δ	Complexity Δ
...query/storage/v1alpha2/stub/BigQueryWriteStub.java	16.66% <16.66%> (ø)	1 <1> (?)
...gquery/storage/v1alpha2/BigQueryWriteSettings.java	18.91% <18.91%> (ø)	2 <2> (?)
...uery/storage/v1beta2/BaseBigQueryReadSettings.java	21.21% <21.21%> (ø)	2 <2> (?)
...igquery/storage/v1beta2/stub/BigQueryReadStub.java	25% <25%> (ø)	1 <1> (?)
...bigquery/storage/v1alpha2/BigQueryWriteClient.java	25.8% <25.8%> (ø)	4 <4> (?)
...1alpha2/stub/GrpcBigQueryWriteCallableFactory.java	37.5% <37.5%> (ø)	3 <3> (?)
.../v1beta2/stub/GrpcBigQueryReadCallableFactory.java	37.5% <37.5%> (ø)	3 <3> (?)
...gquery/storage/v1beta2/BaseBigQueryReadClient.java	50% <50%> (ø)	8 <8> (?)
...y/storage/v1alpha2/stub/GrpcBigQueryWriteStub.java	80% <80%> (ø)	8 <8> (?)
...orage/v1alpha2/stub/BigQueryWriteStubSettings.java	81.57% <81.57%> (ø)	14 <14> (?)
... and 14 more

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