This commit also add a test of this feature.

Note the search JSON output doesn't contain any jobs because they can
not be exported to JSON yet.

The JSON output on a search query matching a build looks like:

```
{
  "builds": [
    {
      "buildoutputs": {
        "out": {
          "path": "/nix/store/wdag3pznrvqk01byk989irg7rq3q2a2c-job"
        }
      },
      "finished": 0,
      "releasename": null,
      "starttime": null,
      "project": "sample",
      "buildproducts": {},
      "timestamp": 1541007629,
      "buildstatus": null,
      "nixname": "job",
      "drvpath": "/nix/store/n9zqndn7j7nyr6gg3bmxvw26cfmdwv2n-job.drv",
      "job": "job",
      "id": 1,
      "stoptime": null,
      "priority": 100,
      "system": "x86_64-linux",
      "jobsetevals": [
        1
      ],
      "jobset": "default",
      "buildmetrics": {}
    }
  ],
  "projects": [],
  "jobsets": [],
  "buildsdrv": []
}
```

Add JSON search API endpoint