feat: support multiple request types and configurable expected status code

[yashvardhan-kukreja]

closes #779

Updated the docs and version of the http-check image as well.
The following are the new configurable environment variables for the http-check

      env:
          - name: CHECK_URL
            value: "https://reqres.in/api/users"
          - name: COUNT                #### default: "0"
            value: "1"
          - name: SECONDS              #### default: "0"
            value: "1"
          - name: PASSING_PERCENT      #### default: "100"
            value: "100"
          - name: REQUEST_TYPE         #### default: "GET"
            value: "POST"
          - name: REQUEST_BODY         #### default: "{}"
            value: '{"name": "morpheus", "job": "leader"}'
          - name: EXPECTED_STATUS_CODE #### default: "200"
            value: "201"

Sample output of a failing check

"kuberhealthy/http": {
      "OK": false,
      "Errors": [
        "unable to retrieve a valid response (expected status: 201) from POST https://reqres.in/api/users checks failed 1 out of 1 attempts"
      ],
      "RunDuration": "39.511730386s",
      "Namespace": "kuberhealthy",
      "LastRun": "2021-01-18T21:48:33.614078779Z",
      "AuthoritativePod": "kuberhealthy-7b5b54ddb7-sggnz",
      "uuid": "e426be13-4a17-4db1-896e-26521f8bb02c"
    }

[yashvardhan-kukreja]

@integrii Please take a look at this PR and let me know if it looks fine.
Thanks!

[integrii]

This looks great! Thank you very much. Always great to see code with comments and end user documentation on the first iteration.