Moving average option for xy series

[flash1293]

Is your feature request related to a problem? Please describe.
For granular time series it's often beneficial to smooth out the data over time.

Describe the solution you'd like
Similar to the fitting function definitions, I would like to be able to specify moving average on a series of an xy chart. For an initial version just specifying the window size and gap policy would be enough:

<AreaSeries {...props} movingAverage={{ mode: 'simple', windowSize: 5, gapPolicy: 'insert_zero' }} />

Describe alternatives you've considered
This can also be solved on the consumer side, but as there are some hairy details around how moving average is implemented and it would be helpful to have a standard implementation for this for all elastic-charts in Kibana.

Additional context

There are some additional details to define, but I don't have a strong opinion on them. I hope we can figure those out using comments on this issue.

Kibana Cross Issues

Checklist

Delete any items that are not applicable to this feature request.

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present

[flash1293]

Quick update on urgency: Based on recent discussions we will probably not use this setting in Lens but use an expression function implementing the same functionality to be able to use it outside of the xy chart type. I removed the kibana cross issue label due to that. I think however it's still a nice feature in general.