docs: add samples for API key auth

[dbolduc]

Part of the work for #14733

Add a sample for apikeys_create_api_key (cloud docs) and apikeys_authenticate_api_key (cloud docs)

These API keys give access to our project's billing account. So we make sure not to print the key_string, and periodically remove old keys that may leak.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 6 lines in your changes missing coverage. Please review.

Project coverage is 93.45%. Comparing base (4868f0a) to head (b41e8b2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
examples/api_key.cc	90.90%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14740      +/-   ##
==========================================
- Coverage   93.46%   93.45%   -0.01%     
==========================================
  Files        2323     2324       +1     
  Lines      208128   208218      +90     
==========================================
+ Hits       194518   194591      +73     
- Misses      13610    13627      +17     

Flag	Coverage Δ
	93.45% <90.90%> (-0.01%)	⬇️

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[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• examples/api_key.cc:17, tag apikeys_create_api_key
• examples/api_key.cc:22, tag apikeys_authenticate_api_key
• examples/api_key.cc:41, tag apikeys_create_api_key
• examples/api_key.cc:70, tag apikeys_authenticate_api_key

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