Extract clients to split repositories

[chingor13]

Does not include bigtable, firestore, or pubsub yet.

[codecov]

Codecov Report

Merging #6894 into master will decrease coverage by 8.61%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6894      +/-   ##
============================================
- Coverage     53.89%   45.28%   -8.62%     
+ Complexity     9096     6249    -2847     
============================================
  Files           712      453     -259     
  Lines         81282    56280   -25002     
  Branches       7307     7229      -78     
============================================
- Hits          43809    25484   -18325     
+ Misses        35521    29138    -6383     
+ Partials       1952     1658     -294

Impacted Files	Coverage Δ	Complexity Δ
...gle/cloud/bigtable/data/v2/BigtableDataClient.java	89.58% <0%> (-1.91%)	32% <0%> (ø)
...eas/src/main/java/io/grafeas/v1/GrafeasClient.java	61.63% <0%> (-1.47%)	59% <0%> (+14%)
...anner/jdbc/ClientSideStatementValueConverters.java	92.22% <0%> (-1.04%)	0% <0%> (ø)
...ogle/cloud/spanner/jdbc/ReadOnlyStalenessUtil.java	70.27% <0%> (-0.97%)	25% <0%> (ø)
...le/cloud/spanner/jdbc/JdbcSqlExceptionFactory.java	45.71% <0%> (-0.67%)	7% <0%> (ø)
.../com/google/cloud/pubsub/v1/MessageDispatcher.java	84.61% <0%> (-0.52%)	23% <0%> (ø)
...ery/datatransfer/v1/DataTransferServiceClient.java	65.35% <0%> (-0.51%)	55% <0%> (+13%)
...e/cloud/bigquery/testing/RemoteBigQueryHelper.java	61.4% <0%> (-0.42%)	6% <0%> (ø)
...able/admin/v2/BaseBigtableInstanceAdminClient.java	58.76% <0%> (-0.4%)	70% <0%> (+16%)
...ava/com/google/cloud/pubsub/v1/OpenCensusUtil.java	71.42% <0%> (-0.24%)	5% <0%> (ø)
... and 410 more

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