Allow creating disabled downloadButton()/downloadLink()

[riccardoporreca]

Including a downloadButton() (similar for downloadLink()) disabled explicitly in the app UI might be convenient for cases where the download is only available after some user interaction.

Before #4041, it was possible to add a "disabled" class when creating a downloadButton() in the UI (and then explicitly re-enable in the app server logic using e.g. shinyjs), but now the extra "disabled" class is always removed:

shiny/srcts/src/bindings/output/downloadlink.ts

Lines 10 to 13 in d84aa94

	el.setAttribute("href", data);
	el.classList.remove("disabled");
	el.removeAttribute("aria-disabled");
	el.removeAttribute("tabindex");

Maybe a nice way to resolve this would be to have built-in support for creating a disabled downloadButton() with disabled = TRUE , similar to actionButton(), with a similar mechanism to updateActionButton() for enabling/disabling it as part of the server logic?