Add the shift model nested in coverage request model

[theosiemensrhodes]

Right now coverage request models only have shiftId. But everytime we fetch we want the shift info too. Make that possible.

[felixma9]

Quick example from meeting:

export type EmbeddedShift = {
id: string;
date: string;
startAt: Date;
endAt: Date;
class: EmbeddedClass;
requestedOn: Date;
instructors: User[];
volunteers: ShiftVolunteer[];
}