[Derived Fields] Add Aggregation support for Derived Fields

[mch2]

Is your feature request related to a problem? Please describe
Now that we have basic derived fields we can add aggregation support for them.

Describe the solution you'd like
Ideally all existing aggregatios will extend to support DerivedField and DerivedField object types.

I have started working on getting this rolling with basic bucket and metric aggs by adding a method to fetch an AggregationScript.LeafFactory directly from DerivedFieldType. This will return a script to be used with valueSources that fetches values from DerivedFieldValueFetcher.

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Describe alternatives you've considered
Ignore agg support for derived fields.