ENH: make `times` parameter of `plot_evoked_joint` more versatile

plot_evoked_joint accepts a times parameter:

https://github.com/mne-tools/mne-python/blob/8df70aea97c42a58d02e1c121679e9c1b2c62869/mne/viz/evoked.py#L1818-L1822

I think the following changes would be an improvement:

• make it possible to determine the number of evenly spaced topos via "auto" (currently hardcoded to 5) and "peaks" (currently hardcoded to 3)
• make it possible to determine a window (or one window per peak) in which "peaks" should be found (currently hardcoded to full time window)

suggested API:

• accept a dict mapping a single str key to value(s):
• if "auto", must be either:
  • an integer bigger than 1, corresponding to the number of evenly spaced topos to be plotted
  • a tuple of the form (n, (tmin, tmax)) to plot n evenly spaced topos in the window (tmin, tmax)
• if "peaks", must be either:
  • an integer bigger than 1, corresponding to the number of peaks (over full time window) to be plotted
  • a tuple of tuples, where each tuple refers to a (tmin, tmax) of a window in which to plot the peak of that window

For example

times = "auto"  # will plot 5 evenly spaced peaks over the whole window
times = np.linspace(0.2, 0.6, 3)  # will plot 3 evenly spaced peaks between 0.2 and 0.6

times = {"peaks": 1}  # will plot 1 single peak within the full time window
times = {"peaks": ((0, 0.1))}  # will plot single peak within 0 to 0.1s window
times = {"peaks": ((0, 0.1), (0.5, 0.8))}  # will plot two peaks within their windows

times = {"mykey": 5}  # error, only "peaks" is an accepted keys

From just thinking about it (haven't looked at the code yet), this should be fairly straight forward, and helpful.

WDYT?

Start in mne/viz/evoked.py around the plot_evoked_joint times handling linked in the issue. Review how the current hardcoded evenly spaced and peak selections are determined, then implement the accepted peaks forms and validate invalid keys or values. Done means the requested windowed peak selection works while existing times usage remains supported.