feat(bigtable): introduce minimal DataConnection implementation

[mpeddada1]

We won't need the empty prepared_query once #15650 is merged.

[codecov]

Codecov Report

❌ Patch coverage is 81.29496% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.11%. Comparing base (f149191) to head (d7d3c03).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
generator/internal/scaffold_generator.cc	0.00%	14 Missing ⚠️
google/cloud/bigtable/data_connection.cc	0.00%	7 Missing ⚠️
...le/cloud/bigtable/internal/data_connection_impl.cc	57.14%	3 Missing ⚠️
google/cloud/bigtable/bound_query_test.cc	96.07%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #15651    +/-   ##
========================================
  Coverage   93.10%   93.11%            
========================================
  Files        2433     2439     +6     
  Lines      223834   223968   +134     
========================================
+ Hits       208402   208539   +137     
+ Misses      15432    15429     -3     

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[mpeddada1]

The compilation error is about unused parameters since the method only returns kUnimplemented at the moment.