refactor: relocate golden files

[dbolduc]

Motivated by #10170

Move the golden files from generator/integration_tests/golden -> generator/integration_tests/golden/v1.

While this is not necessary to test forwarding headers, I think it is best that our golden files reflect our real libraries. (That is, we could leave the golden files where they are and generate forwarding headers in generator/integration_tests/some_random_directory/.... it would work fine, but it would be confusing).

The PR is broken into atomic commits, so hopefully that makes things easier to review.

In the update unit tests commit, I ran the following:

cd generator/integration_tests/tests
sed -i 's;golden/;golden/v1/;' *
sed -i 's;golden::;golden_v1::;' *
sed -i 's;golden_internal::;golden_v1_internal::;' *
sed -i 's;golden_mocks::;golden_v1_mocks::;' *
sed -i 's;namespace golden;namespace golden_v1;' *
sed -i 's;namespace golden_internal;namespace golden_v1_internal;' *
sed -i 's;namespace golden_mocks;namespace golden_v1_mocks;' *

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[codecov]

Codecov Report

Base: 93.91% // Head: 93.91% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (125aa8a) compared to base (0bea867).
Patch coverage: 97.74% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10454   +/-   ##
=======================================
  Coverage   93.91%   93.91%           
=======================================
  Files        1614     1612    -2     
  Lines      146738   146708   -30     
=======================================
- Hits       137805   137782   -23     
+ Misses       8933     8926    -7     

Impacted Files	Coverage Δ
...tion_tests/golden/v1/golden_kitchen_sink_client.cc	80.68% <ø> (ø)
...ation_tests/golden/v1/golden_kitchen_sink_client.h	100.00% <ø> (ø)
...n_tests/golden/v1/golden_kitchen_sink_connection.h	100.00% <ø> (ø)
...lden_kitchen_sink_connection_idempotency_policy.cc	71.42% <ø> (ø)
...ation_tests/golden/v1/golden_thing_admin_client.cc	98.10% <ø> (ø)
...ration_tests/golden/v1/golden_thing_admin_client.h	100.00% <ø> (ø)
...on_tests/golden/v1/golden_thing_admin_connection.h	100.00% <ø> (ø)
...olden_thing_admin_connection_idempotency_policy.cc	95.23% <ø> (ø)
.../v1/internal/golden_kitchen_sink_auth_decorator.cc	70.12% <ø> (ø)
...n/v1/internal/golden_kitchen_sink_auth_decorator.h	100.00% <ø> (ø)
... and 91 more

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