This fixes an issue where the connection would be closed after only a partial body read. The RoundTripper would close the conn before returning the response which included a partially buffered body. This would actually work for short responses, since the backing bufio would do a partial read, but would fail on all but the shortest responses.

Normally connection lifecycle is handled outside the realm of the `RoundTripper`, but for our purposes we don't want to re-use connections. Since it is a requirement that all response bodies be closed, we can piggy-back on top of that to ensure the connection is closed.

Fixes elastic#8588

This commit adds a new `json` check for HTTP responses letting users define an arbitrary condition to match against parsed JSON to determine whether an endpoint is up or down.

The nice thing about structured checks like this is that it makes it easy for users to precisely piggy-back on top of existing JSON endpoints, or write their own where a given key/value could indicate the health of an external system. In a sense, it allows users to write a healthcheck endpoint.

An example can be seen below:

```yaml
heartbeat.monitors:
- type: http
  # List or urls to query
  urls: ["http://localhost:9200"]
  schedule: '@every 10s'
  check.response.json:
    - description: check version
      condition: equals.version.number: "6.4.0"
```