Add a "timestamp" item to the info return value from DrakeGymEnv.step(...)

[samzapo]

It is often helpful to have the simulation timestamp after a call to env.step() to, e.g. render a video of image observations with simulation time as the video's timestamp.

drake/bindings/pydrake/gym/_drake_gym_env.py

Line 209 in 59a15c3

def step(self, action):

[samzapo]

Alternatively, this issue could be titled "add option to customize info returned by calls to step() and reset().

E.g., a Callable provided to __init__.

DrakeGymEnv.__init__(
    ...
    calc_step_info_function :  Callable[[Simulator, Context], dict] = None,
    ...
)

where an implementation that solves my issue would be:

def calc_step_info_function(simulator, diagram_context) -> dict:
    info = {"timestamp", diagram_context.get_time()}
    return info

[jwnimmer-tri]

Given #21900, I'll assign @samzapo on this issue.