Avoid double loading DocValues with ScriptDocValues and Fields API

[stu-elastic]

When implementing the Fields API, we should avoid double loading DocValues if a users uses ScriptDocValues (doc['myfield']) and DocValuesField (field('myfield')).

This will allow users to incrementally use the scripting Fields API in their scripts, when they add functionality or fix a bug, for example without a surprising penalty.

Refs: #78950

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[stu-elastic]

DocValuesField has getScriptDocValues which avoids this double loading.