Add "browsing examples" section to the Welcome Screen

[abey79]

The "browsing examples" section is currently missing from the Welcome Screen:

Goals for the first iteration

• The example list "belongs" to the Welcome Screen (so it's available when the Welcome Screen is, which currently means "as background", when all recordings are closed).
• Example list is built based on a hosted json file containing example metadata.
• Only list examples for which we have a built RRD file (same as with the current demo.rerun.io).
• Display: simple list of example summaries including title, description, tags, and thumbnail.
• Launching an example triggers downloading the RRD from web.
• While downloading the RRD, the existing "loading screen" shows up.

Skip for later

• search bar UI
• filter by tag UI
• download progress bar

TODO

• Generate JSON with example metadata #3105
• support for launching streams: All files Rerun can open should support command line, drag, and open from menu  #2229
• nice "example card UI"
• lazy-loaded thumbnails using Flexible and convenient texture loading emilk/egui#3291

[emilk]

For the image loading we can use a similar strategy as I used in lampsitter/egui_commonmark#8
That is: a map of Url to PollPromise<Result<TextureHandle>>.

At some point I'd like to add proper support for this to egui, and maybe this is the point. egui could support a plugin for converting an URI to a ColorImage, and then implement the cache for that.

Here's a design for this:

• Flexible and convenient texture loading emilk/egui#3291