* Migrate scripted metric aggregation scripts to ScriptContext design #29328
* Rename new script context container class and add clarifying comments to remaining references to params._agg(s)
* Misc cleanup: make mock metric agg script inner classes static
* Move _score to an accessor rather than an arg for scripted metric agg scripts
This causes the score to be evaluated only when it's used.
* Documentation changes for params._agg -> agg
* Migration doc addition for scripted metric aggs _agg object change
* Rename "agg" Scripted Metric Aggregation script context variable to "state"
* Rename a private base class from ...Agg to ...State that I missed in my last commit
* Clean up imports after merge
8e4768890a