Pass aggregator suppliers as aggregator factory constructor arguments

[not-napoleon]

Currently, during aggregator construction, we look up the supplier from the ValuesSourceRegistry twice. Once in ValuesSourceAggregationBuilder#doBuild and then again in the specific factory's doCreateInternal method. Instead, we could just pass the instance we already have from VSAB and streamline doCreateInternal. While we're touching that, we can get rid of the type checking around the suppliers. It was handy when we were writing the registry to see if we wired something up wrong, but now it's just so much boiler plate. Could be replaced with an assert or even just a bare cast.

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)

[Thlamz]

I want to start working on this issue! Please tell me if there is already someone else working on it, I expect to open a PR soon.