Add support for getting attributes on has_many associations (&attributes=vms.name)

[jrafanie]

Because we can't ask for a single attribute on a plural association and if we don't have an exposed virtual attribute, I think this would require us to bring back the whole collection proxy with all of their attributes (all vms with all of their real attributes) in order to get just one or two.

This doesn't work:

/api/providers?expand=resources&attributes=name,vms.name

Results in calling name on the collection proxy, resulting in the name of the collection, not an array of names for each Vm.

    "resources": [
        {
            "href": "http://localhost:3000/api/providers/4",
            "name": "REDACTED1",
            "id": "4",
            "vms": {
                "name": "Vm"
            }
        },
        {
            "href": "http://localhost:3000/api/providers/3",
            "name": "REDACTED2",
            "id": "3",
            "vms": {
                "name": "Vm"
            }
        },

Meanwhile, singular associations work:

/api/vms?expand=resources&attributes=name,hardware.cpu_sockets

    "resources": [
        {
            "href": "http://localhost:3000/api/vms/485",
            "name": "REDACTED3",
            "id": "485",
            "hardware": {
                "cpu_sockets": 1
            }
        },
        {
            "href": "http://localhost:3000/api/vms/338",
            "name": "REDACTED4",
            "id": "338",
            "hardware": {
                "cpu_sockets": 4
            }
        },

Related to #805

[kbrock]

our virtual attributes only really allow you to do a single child attribute. It is because we implement them as sub queries and sql subqeries only handle a single value per row. Maybe we can do better with postgres functions to convert to sql arrays.

[miq-bot]

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