Add public Config.update_from_dirs() method

[schlunma]

Description

While working on making ESMValTool with the new configuration (#2448) I noticed that we are missing a simple public function to update the configuration from one or more directories.

This PR adds that.

Related to #2371

Link to documentation: https://esmvaltool--2538.org.readthedocs.build/projects/ESMValCore/en/2538/api/esmvalcore.config.html#esmvalcore.config.Config.update_from_dirs

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.91%. Comparing base (5b28372) to head (3dcd2d5).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2538      +/-   ##
==========================================
+ Coverage   94.90%   94.91%   +0.01%     
==========================================
  Files         251      251              
  Lines       14295    14301       +6     
==========================================
+ Hits        13566    13574       +8     
+ Misses        729      727       -2     

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[valeriupredoi]

noicee! Just one (possible offbeat) comment from me, can merge right after Manu bothers giving me an answer to that 🍺