[Lens] Support dynamic thresholds based on a query

[wylieconlon]

This feature is the lower priority than the two other types of thresholds, constant value thresholds and thresholds based on already-existing chart data, and is also more technically complex.

There are some use cases where users will want to run a query to determine what the threshold values should be. For example, the threshold could be set using a percentile of the entire time range- showing days where the values are under the 5th percentile.

[elasticmachine]

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

[flash1293]

Adding layer includes a popover for different layer types (threshold being another layer type):

This is how a threshold layer looks like (dimension groups are the axes that can get thresholds):

The header of regular data layers looks a little different as well:

Threshold configuration has different options than a data series (should be straight forward). Data part of the configuration has a separate tab "static value". If a new threshold layer is created, it's preset with a static value at 75% of the current axis (if all information is available)

It's possible to switch to quick function / formula and fetch dynamic data instead of using static data (regular UI in this case)