Allow adding metadata to CellDisplayOutput

[DonJayamanne]

#96433 tracks ability to add metadata to cells, & notebooks.
We also need the ability to add metadata to cell output as well.

Consider the following example:

• When rendering matplotlibs in Jupyter, the output contains additional metadata useful for rendering.
• E.g. if a png/svg is returned metadata can be returned in the output indicating whether a white/light background needs to be used.
• In such a case, a custom renderer for image/png or image/svg+xml will need access to this metadata to wrap the image with a white background container or the like.

@rebornix

[vscodebot]

(Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• Add metadata to Notebook and NotebookCell (#97275)
• Allow adding metadata to NotebookData and NotebookCellData (#97077)

[rebornix]

Re #93265 . We now support metadata (and also custom ones) in CellDisplayOutput.