Adding Scatter Plot functionality to DataTree

[eni-awowale]

Description

This is an enhancement from the archived repository xarray-contrib/datatree#47 and moved upstream #9348.

Would be nice to add additionally functionality for plotting histograms.

Checklist

• Closes Plotting methods #9348
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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