Add Hovmoeller Z vs. time plot to monitoring diagnostic (for Changelog v2.10: authors: @cubeme and @helgehr)

[cubeme]

Description

This PR adds functionality for plotting Hovmoeller z vs. time plots.

• Closes Add Hovmöller Z vs. time plot to diagnostic monitor/multi_datsets.py #3344
• Link to documentation: https://esmvaltool--3345.org.readthedocs.build/en/3345/api/esmvaltool.diag_scripts.monitor/multi_datasets.html#api-esmvaltool-diag-scripts-monitor-multi-datasets

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[schlunma]

Thanks @cubeme and @helgehr, that looks fantastic! I have some very minor comments (if you agree with them, you can just accept the suggestions here on GitHub). Once they are resolved, I will give this a final test and merge 👍

[schlunma]

Tested with actual data, everything looks and works as expected. Thanks @cubeme and @helgehr 🚀