Add render.download

[wch]

This PR adds render.download. This is a proof-of-concept; the final implementation will have to be done after #964 is merged, and will be significantly different. But the app-developer-facing API will be mostly the same.

The existing API is to use session.download, like this:

# UI contains:
ui.download_button("dl", "Download File")

# Server function contains:
@session.download(filename="hello.txt")
def dl():
    yield "Hello, world!\n"

This is quite a bit different from all other outputs, because it relies on the session object. This also doesn't work for Express apps, because there isn't a session object available. (I'll note here that it would be possible to add a session object that could be imported from shiny.express, but the change in this PR is a better one.)

With this PR, the app uses render.download instead of session.download:

# UI contains:
ui.download_button("dl", "Download File")

# Server function contains:
@render.download(filename="hello.txt")
def dl():
    yield "Hello, world!\n"

I think this makes more sense from the app author's perspective.

In an Express app, the code would look like this (the button is automatically inserted in the UI):

@render.download(filename="hello.txt")
def dl():
    yield "Hello, world!\n"

However, there is currently one limitation with this: it would be nice to be able to pass in a label for the button, but it doesn't work, because of where the function is defined for the button. @schloerke With the changes in #964, would it be possible to pass the label to the ui.download_button function?

[wch]

The button_label gets passed here, but it currently isn't used, and doesn't end up in the UI. It would need to somehow go to the call to download_button_wrapped().

[schloerke]

This should work great in #964

[jcheng5]

I think this would work today with:

Suggested change

	def download_button_wrapped(id: str) -> Tag:
	return _ui.download_button(id, label=id)
	@output_transformer(
	default_ui=download_button_wrapped
	) # pyright: ignore[reportGeneralTypeIssues]
	async def DownloadTransformer(
	_meta: TransformerMetadata,
	_fn: DownloadHandler,
	*,
	button_label: Optional[TagChild] = None,
	@output_transformer(
	default_ui=ui.download_button,
	default_ui_passthrough_args=["label"],
	) # pyright: ignore[reportGeneralTypeIssues]
	async def DownloadTransformer(
	_meta: TransformerMetadata,
	_fn: DownloadHandler,
	*,
	label: Optional[TagChild] = "Download",

Definitely will be easier with Barret's new class-based approach.

[wch]

Closing in favor of #977.