[4.x] Add ability to add multiple Resrv enabled items to an entry

[afonic]

Description of the feature:

• One regular Statamic entry, for example "Boat trip" should have an entries field.
• That field will contain multiple Resrv enabled entries, for example "Beneteau", "Lagoon", "Hanse" boats.
• A page loading the "Boat trip" entry should be able to get availability for all three items and display it.
• An index page of boat trips should display any boat trips that have at least one boat available.