feat: add flatten plugin

[stephaniewang526]

add flatten plugin to flatten pom.xml

[codecov]

Codecov Report

Merging #117 into master will increase coverage by 2.02%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #117 +/- ## ============================================ + Coverage 71.86% 73.88% +2.02%  - Complexity 270 319 +49  ============================================ Files 40 42 +2 Lines 1784 2229 +445 Branches 9 70 +61 ============================================ + Hits 1282 1647 +365  - Misses 497 552 +55  - Partials 5 30 +25

Impacted Files	Coverage Δ	Complexity Δ
...google/cloud/bigquery/storage/v1alpha2/Waiter.java	72.54% <0%> (ø)	14% <0%> (?)
.../cloud/bigquery/storage/v1alpha2/StreamWriter.java	81.17% <0%> (ø)	28% <0%> (?)
...igquery/storage/v1alpha2/ProtoSchemaConverter.java	93.54% <0%> (+0.21%)	6% <0%> (ø)	⬇️
...y/storage/v1alpha2/stub/GrpcBigQueryWriteStub.java	93.69% <0%> (+0.9%)	12% <0%> (+1%)	⬆️
...bigquery/storage/v1alpha2/BigQueryWriteClient.java	61.29% <0%> (+9.67%)	14% <0%> (+2%)	⬆️
...gquery/storage/v1alpha2/BigQueryWriteSettings.java	29.72% <0%> (+10.81%)	6% <0%> (+4%)	⬆️

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[chingor13]

If this works well for us here, we should move all these configs into the shared-config parent.