WIP Adding support for TF structure for arbitrary many watermarks

[aubrey-stripe]

Motivation: Allow for arbitrarily many watermarks in a chart.

Tested that this allows for a structure like the following in a dashboard chart

axis_right {
  high_watermark = 0.99
  high_watermark_label = "bad"
  low_watermark = 0.01
  low_watermark_label = "good-ish"
  watermarks {
    label = "bad"
    value = 0.99
  }
  watermarks {
    label = "good-ish"
    value = 0.01
  }
}

~ module.dashboards.signalform_time_chart.chart_133767_0
    axis_right.#:                                       "0" => "1"
    axis_right.509551474.high_watermark:                "" => "0.99"
    axis_right.509551474.high_watermark_label:          "" => "bad"
    axis_right.509551474.low_watermark:                 "" => "0.01"
    axis_right.509551474.low_watermark_label:           "" => "good-ish"
    axis_right.509551474.watermarks.#:                  "0" => "2"
    axis_right.509551474.watermarks.2403508488.label:   "" => "bad"
    axis_right.509551474.watermarks.2403508488.value:   "" => "0.99"
    axis_right.509551474.watermarks.3497209033.label:   "" => "good-ish"
    axis_right.509551474.watermarks.3497209033.value:   "" => "0.01"