Extract gcloud-maven-plugin, bigtable-emulator

[chingor13]

• gcloud-maven-plugin extracted to https://github.com/googleapis/java-gcloud-maven-plugin
• bigtable-emulator extracted to https://github.com/googleapis/java-bigtable-emulator

[codecov]

Codecov Report

Merging #6452 into master will increase coverage by 0.36%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6452      +/-   ##
============================================
+ Coverage     45.98%   46.34%   +0.36%     
- Complexity    26495    27990    +1495     
============================================
  Files          2613     2613              
  Lines        282539   287955    +5416     
  Branches      33554    33751     +197     
============================================
+ Hits         129924   133453    +3529     
- Misses       143358   144282     +924     
- Partials       9257    10220     +963

Impacted Files	Coverage Δ	Complexity Δ
...ud/devtools/containeranalysis/v1/GrafeasUtils.java	0% <0%> (-90.91%)	0% <0%> (-1%)
.../containeranalysis/v1/ContainerAnalysisClient.java	47.36% <0%> (-8.41%)	15% <0%> (+1%)
...va/com/google/cloud/compute/v1/InstanceClient.java	55.09% <0%> (-7.79%)	147% <0%> (+36%)
...onitoring/v3/NotificationChannelServiceClient.java	64.49% <0%> (-7.13%)	43% <0%> (+10%)
...ava/com/google/cloud/compute/v1/ProjectClient.java	57.07% <0%> (-6.99%)	55% <0%> (+13%)
.../com/google/cloud/compute/v1/TargetPoolClient.java	57.73% <0%> (-6.98%)	47% <0%> (+11%)
...va/com/google/cloud/compute/v1/SnapshotClient.java	55.73% <0%> (-6.88%)	31% <0%> (+7%)
...oogle/cloud/kms/v1/KeyManagementServiceClient.java	62.88% <0%> (-6.84%)	103% <0%> (+25%)
...com/google/cloud/compute/v1/ReservationClient.java	59.14% <0%> (-6.83%)	39% <0%> (+9%)
...a/com/google/cloud/compute/v1/NodeGroupClient.java	60.36% <0%> (-6.82%)	51% <0%> (+12%)
... and 648 more

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