Implement HTMLSelectAndOtherField and HTMLSelectOrOtherField in OOUIHTMLForm.
Yeah, HTMLSelectOrOtherField has low usage and two alternatives, so we could probably get away with skipping it. But I think most of the code will be reused from HTMLSelectAndOtherField.
I think both of these are basically just a dropdown plus a textfield? It shouldn't be difficult to implement. We should just have a OOUI\Widget subclass that would display the two fields side-by-side, similar to e.g. MediaWiki\Widget\ComplexTitleInputWidget / mw.widgets.ComplexTitleInputWidget. This is needed because HTMLFormField uses OOUI\FieldLayout, which expects a single widget (HTMLFormField::getInputOOUI() is technically allowed to return HTML strings, which will be dynamically wrapped in a Widget, but that's hacky and I'm not sure if we actually use it anywhere.)