refactor(common): external account format parser

[coryan]

I will need this for other subject token sources, including URL-based ones.

Part of the work for #5915

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[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Base: 93.84% // Head: 93.84% // No change to project coverage 👍

Coverage data is based on head (7d35ce8) compared to base (7d35ce8).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10281   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files        1575     1575           
  Lines      144311   144311           
=======================================
  Hits       135424   135424           
  Misses       8887     8887           

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