fix(bigtable): the mocks library is no longer header-only

[coryan]

The library is no longer header-only, it needs to become a real library.

Part of the work for #5782

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This change is 

[google-cloud-cpp-bot]

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[dbolduc]

Thanks

[codecov]

Codecov Report

Merging #9568 (9efd4ee) into main (29ff9d3) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #9568   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files        1491     1491           
  Lines      137643   137643           
=======================================
+ Hits       129900   129904    +4     
+ Misses       7743     7739    -4     

Impacted Files	Coverage Δ
google/cloud/internal/curl_rest_client.cc	96.85% <0.00%> (-0.79%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (ø)
google/cloud/pubsub/samples/samples.cc	90.77% <0.00%> (+0.07%)	⬆️
google/cloud/storage/parallel_upload.cc	98.61% <0.00%> (+0.34%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
...loud/bigtable/internal/connection_refresh_state.cc	98.61% <0.00%> (+2.77%)	⬆️

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