* Scripted metric aggregations: add deprecation warning and system property to control legacy params
Scripted metric aggregation params._agg/_aggs are replaced by state/states context variables. By default the old params are still present, and a deprecation warning is emitted when Scripted Metric Aggregations are used. A new system property can be used to disable the legacy params. This functionality will be removed in a future revision.
* Fix minor style issue and docs test failure
* Disable deprecated params._agg/_aggs in tests and revise tests to use state/states instead
* Add integration test covering deprecated scripted metrics aggs params._agg/_aggs access
* Disable deprecated params._agg/_aggs in docs integration tests and revise stored scripts to use state/states instead
* Revert unnecessary migrations doc change
A relevant note should be added in the changes destined for 7.0; this PR is going to be backported to 6.x.
* Replace deprecated _agg param bwc integration test with a couple of unit tests
* Fix compatibility test after merge
* Rename backwards compatibility system property per code review feedback
* Tweak deprecation warning text per review feedback
* 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
