feat(bigtable): instance name as a class

[dbolduc]

Part of the work for #9348

Plagiarized from: spanner/instance.h

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9374 (8f820a0) into main (f91081c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9374   +/-   ##
=======================================
  Coverage   94.64%   94.64%           
=======================================
  Files        1471     1474    +3     
  Lines      135321   135379   +58     
=======================================
+ Hits       128074   128131   +57     
- Misses       7247     7248    +1     

Impacted Files	Coverage Δ
google/cloud/bigtable/instance_resource.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/instance_resource.h	100.00% <100.00%> (ø)
google/cloud/bigtable/instance_resource_test.cc	100.00% <100.00%> (ø)
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.22% <0.00%> (-0.70%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (-0.25%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.11% <0.00%> (-0.08%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	88.55% <0.00%> (+1.01%)	⬆️

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[coryan]

A small nit in the comments.

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