Data validation: validate `Array` of `object`s have a specific structure

[humitos]

I found myself want to validate the object items included into an Array. However, I didn't find a quick way to do it while working on #281 so I wrote a TODO comment there.

A small example of what I want is on the versions.active field:

    versions: {
      type: "object",
      required: ["current", "active"],
      properties: {
        active: {
          type: "array",
          // TODO: validate each item of the array has the following structure
          //
          // items: { type: "object" },
          // required: ["slug", "urls"],
          // properties: {
          //   slug: { type: "string" },
          //   urls: {
          //     type: "object",
          //     required: ["documentation"],
          //     properties: {
          //       documentation: { type: "string" },
          //     },
          //   },
          // },
        },