Recipe - Stratosphere - Autoassess diagnostics

Implementing [Stratosphere - Autoassess recipe](https://docs.esmvaltool.org/en/latest/recipes/recipe_autoassess_stratosphere.html)

Missing obs data:

`ta, ua, hus` - ERA-Interim

Start with the linked Stratosphere–Autoassess recipe and the repository's existing recipe or workflow definitions; trace how observational inputs are declared. Check how ERA-Interim data are handled for ta, ua, and hus, then validate that the recipe can run with those inputs and produces the expected assessment output.

Written by the indexing model from the issue text.