Add attributes as code cells option (alongside classes)

[mcanouil]

From #5456 and yihui/knitr#2256, I believe it would be very useful to be able to add any kind of attributes and not only classes to cell container.
For instance, knitr offers class-* and the general attr-* options to customise code cells.

This attributes could help leverage many Bootstrap features already embedded in Quarto, such as popover or tooltip which uses data-bs-* attributes.

[mcanouil]

@dragonstyle (cc @cscheid) do you think that's something easily achievable? I believe it could be a nice complementary/general option to classes which will completely avoid the need to put code cell into fenced divs to add attributes (when using engine: knitr, there are attr-*, but nothing for Jupyter).

[cderv]

I think this should be feasible to add attributes to the cell enveloppe like we do with classes

Probably harder part would be the interaction with attr-* from knitr in case both are provided. Probably something to handle.

[mcanouil]

Closing in favour of #6253