[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 #19562 (a8342d3) into release-2.8.0 (206c5b8) will decrease coverage by 3.18%.
Report is 2 commits behind head on release-2.8.0.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           release-2.8.0   #19562      +/-   ##
=================================================
- Coverage          70.53%   67.35%   -3.18%     
=================================================
  Files                749      984     +235     
  Lines              93978   107090   +13112     
  Branches               0     2671    +2671     
=================================================
+ Hits               66284    72127    +5843     
- Misses             24107    31082    +6975     
- Partials            3587     3881     +294     

Flag	Coverage Δ
unittests	67.35% <ø> (-3.18%)	⬇️

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

... and 241 files with indirect coverage changes

[chlins]

lgtm