[Lens] Support dynamic thresholds based on data from the chart

[wylieconlon]

This issue depends on having constant value thresholds first, and it's lower priority. Dynamic thresholds use pre-defined functions based on the values shown in the chart, such as:

• Overall average
• Overall median
• Overall sum
• Overall min

But dynamic thresholds are not based on additional data, which is a separate feature.

These features can be used when comparing multiple series to see which series are more "normal", or to put the values into context.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[dej611]

Closed via #108342

[monfera]

The word "threshold" has quite specific connotations. For example, the example shown above (an average line) is absolutely and positively not a threshold. An appropriate, less specific and fairly standard industry term would be, "reference line"

Please let's not release it with the misleading "Threshold" on the UI. I'd also like to ask Datavis reviewing in user facing datavis related PRs

cc @ghudgins @markov00