feat(bigquery): add migration service

[coryan]

Fixes #10021

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This change is 

[google-cloud-cpp-bot]

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[google-cloud-cpp-bot]

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For commit: d193ba217c9e4640f4f1be2da6bfc06207795365

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[codecov]

Codecov Report

Base: 94.22% // Head: 94.22% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d69e588) compared to base (a4d7937).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10034   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files        1522     1522           
  Lines      141402   141402           
=======================================
+ Hits       133232   133237    +5     
+ Misses       8170     8165    -5     

Impacted Files	Coverage Δ
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (-0.67%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
google/cloud/storage/internal/curl_handle.cc	77.90% <0.00%> (-0.56%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.70% <0.00%> (-0.08%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.65% <0.00%> (+0.83%)	⬆️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.11%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
google/cloud/internal/async_connection_ready.cc	93.61% <0.00%> (+4.25%)	⬆️

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[dbolduc]

Do you want to build the library?

google-cloud-cpp/google/cloud/bigquery/CMakeLists.txt

Lines 32 to 33 in a4d7937

	add_library(
	google_cloud_cpp_bigquery # cmake-format: sort

[coryan]

Do you want to build the library?

google-cloud-cpp/google/cloud/bigquery/CMakeLists.txt

Lines 32 to 33 in a4d7937

	add_library(
	google_cloud_cpp_bigquery # cmake-format: sort

picky picky picky ...

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: d69e588820c43d0d948b11907ccc9a1be65303e0

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[coryan]

PTAL