[APM] Latency correlations: Display skeleton loaders in place of chart and table for initial loading experience

[formgeist]

Summary

We should consider improving the initial loading experience when the user opens the flyout to feature skeleton loaders of the chart and table to indicate what content is coming once the initial load is complete and data is starting to show in the chart and table.

This is for both options, but it's most prevalent in the Latency correlations experience.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[sorenlouv]

Correlations is no longer in a flyout. Skeletons in general is still something we could consider.