[datadog_powerpacks] support for query table

[dashashifrina]

This pull request implements the second part of the support for the powerpack resource in Terraform.

Previous PR:

• [datadog_powerpacks] Implement support for basic widgets #2157
• [datadog_powerpacks] Implement support for live span and more widgets #2158
• [datadog_powerpacks] implement support for nine more widgets #2161

Changes

• Add support for query_table
• Add test and recorded cassettes for all the widgets listed above.
• Add doc.

How to test:

1. Build the provider locally.
2. Try this powerpack resource with all the supported widgets:

resource "datadog_powerpack" "test_powerpack" {
  name         = "{{uniq}}"
  tags = ["tag:foo1"]
  description   = "Created using the Datadog provider in Terraform"
  template_variables {
    defaults = ["defaults"]
    name     = "datacenter"
  }
  widget {
    query_table_definition {
      title_size = "16"
      title = "system.cpu.user, system.load.1"
      title_align = "right"
      request {
      aggregator = "max"
      conditional_formats {
        palette = "white_on_green"
        value = 90
        comparator = "<"
      }
      conditional_formats {
        palette = "white_on_red"
        value = 90
        comparator = ">="
      }
      q = "avg:system.cpu.user{account:prod} by {service, team}"
      alias = "cpu user"
      limit = 25
      order = "desc"
      cell_display_mode = ["number"]
    }
      request {
        q = "avg:system.load.1{*} by {service, team}"
        aggregator = "last"
        conditional_formats {
          palette = "custom_bg"
          value = 50
          comparator = ">"
        }
        alias = "system load"
        cell_display_mode = ["number"]
      }
      custom_link {
        link = "https://app.datadoghq.com/dashboard/lists"
        label = "Test Custom Link label"
      }
      custom_link {
        link = "https://app.datadoghq.com/dashboard/lists"
        is_hidden = true
        override_label = "logs"
      }
      has_search_bar = "auto"
    }
}

  widget {
    query_table_definition {
      request {
        apm_stats_query {
        service = "service"
        env = "env"
        primary_tag = "tag:*"
        name = "name"
        row_type = "resource"
        }
      }
    has_search_bar = "never"
    }
  }
  widget {
    query_table_definition {
      request {
        formula {
          formula_expression = "query1"
          limit {
            count = 500
            order = "desc"
          }
          conditional_formats {
            palette = "white_on_green"
            value = 90
            comparator = "<"
          }
          conditional_formats {
            palette = "white_on_red"
            value = 90
            comparator = ">="
          }
        }
        query {
          metric_query {
          data_source = "metrics"
          query       = "avg:system.cpu.system{*} by {datacenter}"
          name        = "query1"
          aggregator  = "sum"
          }
        }
      }
    }
  }
  widget {
    query_table_definition {
      request {
        query {
          apm_dependency_stats_query {
            name           = "my-query"
            data_source    = "apm_dependency_stats"
            env            = "ci"
            service        = "cassandra"
            operation_name = "cassandra.query"
            resource_name  = "CREATE TABLE IF NOT EXISTS foobar"
            stat           = "avg_duration"
          }
        }
      }
    }
  }
}