Add simulation time and rate (ala show_time) to meshcat

[jwnimmer-tri]

In our drake_visualizer, we have a little footer in the bottom of the window that shows the simulation clock, and its realtime rate (vs the wall clock). We should offer the same in MeshcatVisualizer as well as meldis.

If I recall correctly, the challenge is that 2d text still needs to be anchored to a 3d point / pose in MeshCat. We'll (probably?) need a way to anchor it to the viewport, instead.

If the anchoring is too difficult, maybe it could be something like a read-only slider (that changes to reflects the observed rate) in the control panel.

Here's our TODO list beyond #16981:

• Add realtime rate chart to meldis #17661
• Display the simulation time (i.e., elapsed time).
• Add smoothing to Meshat realtime rate display #21512
• Add tooltips (or similar) to summarize each timing plot option, so users can discover what the plots mean.
• Reduce the brittleness of how we add custom type-handlers on the js side.