Add json output for the search API endpoint
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": []
}
```
0d2a2d8923