re_datastore: arrow table debug-view

[emilk]

A birds-eye view of our inner table/query engine.

This is a nice panel for looking into the main table(s) and seeing their columns etc. Good for debugging, but also shows the user data in its most naked form.

This is in aid of #393

[teh-cmc]

adding the notes I took while writing the MVP:

Data store GUI browser

• the store currently provides a very thorough Display implementation that makes it manageable to keep track of what's going on internally
• it'd be even better to have something similar but as an interactive UI panel, akin to a SQL browser

[jondo2010]

My initial "imagine what this would look like" is an interactive/collapsible version of the stdout debug output from the store.

[emilk]

This can now be implemented as a separate space view class. However, it makes very little sense unless we also have the UI to select/switch between different space view classes.

[emilk]

Some inspiration:

• examples/rust/custom_space_view
• examples/rust/extend_viewer_ui/src/main.rs
• Together with egui_extras::TableBuilder (see usage in the current code)

[abey79]

A very short, immediately useful thing could be a Dump datastore command in the Command palette that would eprintln! the datastore.

I'm currently sticking this in App::ui():

        if let Some(ctx) = store_context {
            if let Some(recording) = ctx.recording {
                eprintln!("{}", recording.entity_db.data_store);
            }
        }