Support any type of dashboard widget by JSON string

[suzuki-shunsuke]

#227

The field json_configuration is added to graylog_dashboard_widget to support any type of dashboard widget.
The value of json_configuration should be JSON string.

Please see the example.

resource "graylog_dashboard_widget" "stacked_chart" {
  description = "stacked chart"
  dashboard_id = graylog_dashboard.test.id
  type         = "STACKED_CHART"
  cache_time = 10
  json_configuration = <<EOF
{
  "interval": "hour",
  "timerange": {
    "type": "relative",
    "range": 86400
  },
  "renderer": "bar",
  "interpolation": "linear",
  "series": [
    {
      "query": "",
      "field": "AccessMask",
      "statistical_function": "count"
    }
  ]
}
EOF
}

[codecov]

Codecov Report

Merging #228 into master will decrease coverage by 0.11%.
The diff coverage is 23.52%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
- Coverage   52.83%   52.71%   -0.12%     
==========================================
  Files         125      125              
  Lines        5962     5994      +32     
==========================================
+ Hits         3150     3160      +10     
- Misses       2620     2642      +22     
  Partials      192      192

Impacted Files	Coverage Δ
dashboard_widget.go	48.48% <100%> (+4.73%)	⬆️
terraform/graylog/resource_dashboard_widget.go	47.49% <18.75%> (-1.51%)	⬇️

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