[BE] Space reservations field 일급컬렉션으로 만든다

[sakjung]

기능 상세

• AS-IS: List<Reservaiton>

@OneToMany(mappedBy = "space", cascade = CascadeType.REMOVE, fetch = FetchType.LAZY, orphanRemoval = true)
@Builder.Default
private List<Reservation> reservations = new ArrayList<>();

• TO-BE: Reservations 일급컬렉션

# 참고!!
@Embedded
@Builder.Default
private Settings spaceSettings = new Settings();