Add support for nested JSON response

In some APIs, such as [AbuseIPDB](https://docs.abuseipdb.com/#check-endpoint), the JSON response has some nested data. There's no way to currently access nested values via our custom API configurations. The proposed syntax is to add support for dot notation of nested information.

For example, given the following JSON response:

```json
{
"data": {
"ipAddress": "118.25.6.39",
"isPublic": true,
"ipVersion": 4,
"isWhitelisted": false,
"abuseConfidenceScore": 100,
"countryCode": "CN",
"countryName": "China",
"usageType": "Data Center/Web Hosting/Transit",
"isp": "Tencent Cloud Computing (Beijing) Co. Ltd",
"domain": "tencent.com",
"hostnames": [],
"totalReports": 1,
"numDistinctUsers": 1,
"lastReportedAt": "2018-12-20T20:55:14+00:00",
"reports": [
{
"reportedAt": "2018-12-20T20:55:14+00:00",
"comment": "Dec 20 20:55:14 srv206 sshd[13937]: Invalid user oracle from 118.25.6.39",
"categories": [
18,
22
],
"reporterId": 1,
"reporterCountryCode": "US",
"reporterCountryName": "United States"
}
]
}
}
```

A `key` of `data.usageType` would allow us to access the "Data Center/Web Hosting/Transit" value.