Add public `Config.update_from_dirs()` method

[schlunma]

Description

While working on making ESMValTool with the new configuration (https://github.com/ESMValGroup/ESMValCore/pull/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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Checklist

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• [x] 🧪 The new functionality is relevant and scientifically sound
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• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🧪 and 🛠 Documentation is available
• [x] 🛠 Unit tests have been added
• [x] 🛠 Changes are backward compatible
• [x] 🛠 Any changed dependencies have been added or removed correctly
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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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