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%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e41dd0a...d90fe3e. Read the comment docs.

[chingor13]

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