refactor(storage): move ProjectTeam to a public header

[coryan]

Part of the work for #8929

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 714d034a82386a260cc8d18420dcf83bc3fb60bd

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Base: 94.20% // Head: 94.20% // No change to project coverage 👍

Coverage data is based on head (714d034) compared to base (f6e9229).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9906   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files        1494     1495    +1     
  Lines      140166   140166           
=======================================
  Hits       132042   132042           
  Misses       8124     8124           

Impacted Files	Coverage Δ
...gle/cloud/storage/internal/access_control_common.h	100.00% <ø> (ø)
google/cloud/storage/project_team.h	100.00% <100.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.68%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	98.31% <0.00%> (+0.56%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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