Add support for transforms on multicategory axes

[etpinard]

Follow-up issue from #3300

[MrQubo]

I think this cannot be done without rewriting hell of a code.
Correctly describing x and y attributes in src/traces/scatter/attributes.js would probably be sufficient for transforms to work with multicategory axes.
But those attributes sometimes can be data_arrays and sometimes can be arrays of data_arrays and I think that current format doesn't allow us to describe this dual nature of those attributes.
In my opinion we should rewrite multicategory axes itself and instead of reusing x and y attributes add new attributes, like multix and multiy and use a data format similar to dimensions attribute in parcats.

[gvwilson]

Hi - this issue has been sitting for a while, so as part of our effort to tidy up our public repositories I'm going to close it. If it's still a concern, we'd be grateful if you could open a new issue (with a short reproducible example if appropriate) so that we can add it to our stack. Cheers - @gvwilson