Support loading multiple http:// and ws:// streams at once with rerun

[emilk]

In #2060 we added support for loading multiple .rrd files at once using rerun foo.rrd bar.rrd. However, this makes us a bit inconsistent, as we do NOT support rerun http://example.com/foo.rrd http://example.com/bar.rrd

The major things we need to accomplish is:

• We must support having multiple channels as input to an App, and make LogDb::data_source as set. This would be a nice cleanup otherwise
• How we handle ArgumentCategory::WebSocketAddr needs to change. In particular adding support for multiple web-viewers will take a quite a bit of work.
• Replace re_smart_channel::Source::Files with just multiple sources over multiple channels

[emilk]

Closed in favor of #2229