refactor(oauth2): prepare for API key auth

[dbolduc]

Part of the work for #14759

Move AuthorizationHeader(...) from a standalone function into the oauth2_internal::Credentials class. This will allow us to return the x-goog-api-key: ... header in the case of API key credentials.

Also generalize the name to AuthenticationHeader, now that we are not exclusively setting the Authorization: ... header.

Note that we cannot change any public interfaces in google::cloud::storage::oauth2. These are sorta-kinda public.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.27%. Comparing base (67b10ea) to head (088f5a8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...cloud/storage/oauth2/authorized_user_credentials.h	50.00%	1 Missing ⚠️
...cloud/storage/oauth2/service_account_credentials.h	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14780      +/-   ##
==========================================
- Coverage   93.27%   93.27%   -0.01%     
==========================================
  Files        2317     2317              
  Lines      208171   208168       -3     
==========================================
- Hits       194176   194168       -8     
- Misses      13995    14000       +5     

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[scotthart]

Reviewable status: 0 of 14 files reviewed, 1 unresolved discussion (waiting on @dbolduc)

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google/cloud/internal/oauth2_credentials.h line 123 at r1 (raw file):

   * HTTP requests.
   */
  virtual StatusOr<std::pair<std::string, std::string>> AuthenticationHeader(

Does this need to be virtual?

[dbolduc]

Does this need to be virtual?

I plan to override it in a new oauth2_internal::ApiKeyCredentials class

[scotthart]

Reviewed 14 of 14 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @dbolduc)