/healthz endpoint accepts JSON now

[farazdagi]

Resolves #5546

• Extends /healthz to accept JSON as preferred output.
• By default text/plain context type is used, however, when JSON is accepted as response (that's when Accept: application/json, */*;q=0.5 header is sent), output is JSON:

{
    "data": [
        {
            "error": "",
            "service": "*attestations.Service",
            "status": true
        },
        {
            "error": "",
            "service": "*powchain.Service",
            "status": true
        }
    ],
    "error": ""
}

• Fixes the response code as well. Previously whenever some service was down HTTP 500 error was returned, this is incorrect use of the code (that code is suitable for situations when server cannot complete request due to some problem, in our case server is capable of performing request, and as such 200 OK is valid code). Each individual service has status field (both in plain and JSON output), as well as error message, so individual services can be checked.
• To keep PR focused, only /healthz endpoint is extended (while code was written in a way that it shouldn't be to hard to extend /metrics and /goroutinez endpoints as well)

[prestonvanloon]

What if the header is unset, as it usually is?

[farazdagi]

Then negotiateContentType() defaults to text/plain (and this is covered by unit test). So, whenever nothing is specified, we basically have the previous functionality, if "Accept:" header is provided, then we parse and use it.