[data.search.aggs]: Create expression functions for object types used in agg params

[lukeelmers]

There are a few agg types which have params which are objects. Currently in the expression functions for each agg type, we handle these params by passing the objects as stringified JSON (which is not ideal)

Instead each arg provided for the agg params should be a subexpression using a dedicated function which returns the object in the correct shape.

Here's the current list of agg params which are stringified objects that would need to be converted:

• timeRange (will be covered in [data.query.timefilter] Create expression function for timerange & add toAst method #67897)
• extended_bounds
• ranges
• geo_bounding_box
• filters
• ...check for any others that are missing from this list

Note that each of these params will also need to add an toExpressionAst method to ensure calling AggConfig.toExpressionAst() returns the expected value

Related to #46902

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)