[pickers] Update toolbar spacing and design

[LukasTy]

Created from #8851 (comment)

We have done some spacing tweaks that emphasize the issue of our picker toolbar spacing.

The current design already has some issues:

• Some touch targets are way too small (i.e. year view button, meridiem (am/pm) buttons)
  
• The DateTimePicker toolbar is the biggest offender as it already has issue of not supporting/fitting all views. Adding seconds views breaks the layout. What will we do when we will have to handle milliseconds as well?
  

Challenges:

• We can't just add space-between date calendar days to achieve consistent spacing, because someone might already rely on the fact, that the margin can even be disabled via the disableMargin prop.
• We should not forget that we also have the option to toggle orientation="landscape" and that should look at least somewhat ok. Although, we can probably continue having exceptions on it, like not supporting it if the seconds view is specified.

Ideas

• We are already going to have a difference between the desktop toolbar after [pickers] Add DigitalClock to DesktopDateTimePicker #8946. So, we can keep this idea and introduce different Toolbar variants for all the pickers if that's necessary.

[oliviertassinari]

I think that it would be great to start exploring moving to Material Design v3, this issue could help us take steps there.