[ML] Add metric field percentile info to file structure endpoint

[peteharverson]

The ML index-based data visualizer displays a frequency density histogram to chart the distribution of values for a metric field. In order to make this chart available in the file data visualizer requires the file structure endpoint to return percentiles data, in addition to the min, max and median values that it currently returns.

As a nice-to-have there could be some way to specify the percentiles to return - the index data visualizer for example requests percentiles with a 5% spacing.

cc @tveasey

[elasticmachine]

Pinging @elastic/ml-core