feat(function): deprecate function_token and document new auth with IAM

[Mia-Cross]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 1.67%. Comparing base (f826d8c) to head (eebb6c7).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/services/function/token.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #3534      +/-   ##
=========================================
- Coverage    1.67%   1.67%   -0.01%     
=========================================
  Files         428     428              
  Lines       46746   46747       +1     
=========================================
- Hits          785     783       -2     
- Misses      45873   45875       +2     
- Partials       88      89       +1     

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