[cherry-pick] Allow POST method to request service/token in readonly mode

[stonezdj]

fixes #18243

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Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

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[codecov]

Codecov Report

Merging #19563 (63fc27f) into release-2.9.0 (6f85d59) will increase coverage by 0.03%.
Report is 21 commits behind head on release-2.9.0.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           release-2.9.0   #19563      +/-   ##
=================================================
+ Coverage          67.38%   67.41%   +0.03%     
=================================================
  Files                993      993              
  Lines             108921   108932      +11     
  Branches            2752     2752              
=================================================
+ Hits               73396    73440      +44     
+ Misses             31580    31541      -39     
- Partials            3945     3951       +6     

Flag	Coverage Δ
unittests	67.41% <ø> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/core/middlewares/middlewares.go	0.00% <ø> (ø)

... and 7 files with indirect coverage changes

[chlins]

lgtm