Add a function for writing recipes in a standard YAML format

[bouweandela]

Description

Add a function for writing recipes in a standardized way. This is helpful when building recipes using a script, see e.g. the scripts in ESMValGroup/ESMValTool#4199 (comment). I have added the function to the public API though the esmvalcore.experimental.recipe.Recipe.to_yaml method for now.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.26%. Comparing base (ef355e2) to head (4ef3e68).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3143      +/-   ##
==========================================
+ Coverage   96.24%   96.26%   +0.01%     
==========================================
  Files         273      274       +1     
  Lines       16172    16243      +71     
==========================================
+ Hits        15565    15636      +71     
  Misses        607      607              

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