feat(bigtable): introduce DataConnection

[dbolduc]

Part of the work for #8860

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9323 (78ffae1) into main (ff90f3e) will increase coverage by 0.00%.
The diff coverage is 48.38%.

@@           Coverage Diff           @@
##             main    #9323   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files        1470     1470           
  Lines      134686   134685    -1     
=======================================
+ Hits       127419   127421    +2     
+ Misses       7267     7264    -3     

Impacted Files	Coverage Δ
google/cloud/bigtable/data_connection.h	100.00% <ø> (ø)
...gle/cloud/bigtable/internal/data_connection_impl.h	100.00% <ø> (ø)
google/cloud/bigtable/mocks/mock_data_connection.h	100.00% <ø> (ø)
...le/cloud/bigtable/testing/table_integration_test.h	68.18% <ø> (ø)
google/cloud/bigtable/data_connection.cc	16.98% <11.11%> (ø)
google/cloud/bigtable/table.h	99.22% <100.00%> (ø)
...e/cloud/bigtable/testing/table_integration_test.cc	80.87% <100.00%> (ø)
...ogle/cloud/bigtable/tests/data_integration_test.cc	97.72% <100.00%> (ø)
...gtable/tests/table_sample_rows_integration_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	97.22% <0.00%> (-0.70%)	⬇️
... and 6 more

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 78ffae1c5c31faa02c058a28931a7c44250d2375

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