Feature: Ability to connect or create if entity doesn't exist

[VojtaStanek]

Here is an example:

@Unique('segment', 'parent')
class Url {
	segment = def.stringColumn().notNull()
	parent = def.manyHasOne(Url)
}

Let's say I would like to create following structure, but I don't know wheather Url(segment="bar") exists:

{
	"segment": "foo",
	"parent": {
		"segment": "bar",
		"parent": null
	}
}

Currently I need one query to check if it exists and then do the mutation. I propose following option:

mutation {
	createUrl(data: {
		segment: "foo",
		parent: {
			connectOrCreate: {
				segment: "bar"
				parent: null
			}
		}
	}) {
		ok
	}
}

Inside connectOrCreate would be also helpful to be able to specify other field. Then the engine could try to insert it as new row, but when unique constraint would be violated it would do "connect" instead.