[scheduler][styled] Add a "color" property to the `CalendarEvent` interface

[flaviendelangle]

Follow up on #17718 and #17704

When the events passed to the <EventCalendar /> component (or any Joy UI component with this prop) have a color prop, this color should take precedence over the resource's color.

In this PR:

• Add the color property
• Apply the color to the event rendered in all the views
• Create a UI in the editing popover to select a color from a hardcoded list provided by us