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

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.

reachweb / statamic-resrv