Add missing python bindings for MeshcatAnimation

[yangwill]

Is your feature request related to a problem? Please describe.
When I use the MeshcatVisualizer from pydrake and I want to only record a short segment of the visualizer to a MeshcatAnimation, the MeshcatAnimation starts at time = 0 instead of the Simulator time when I call StartRecording().

Describe the solution you'd like
I see that there's a set_start_time()function in MeshcatAnimation to configure the start time, but there is no corresponding python binding in pydrake. I would like the c++ function for MeshcatAnimation to have corresponding python bindings to have access to the features in MeshcatAnimation.

Describe alternatives you've considered

Additional context

[jwnimmer-tri]

It's probably worth doing both start_time() and set_start_time() at the same time. Both seem to have been overlooked in #15937.