Mark Foreach Processor as experimental

[talevy]

From an offline discussion with @martijnvg and @BigFunger...

The foreach processor breaks many of the conventions of the other processors in Ingest.
It allows for a nested processor to execute in a separate context, and this separate context has been proven to be leaky.

Issues like #19601 and #19592 poke holes in the current interface we have designed. For these reasons, I think that the processor should be marked experimental for the upcoming 5.0 release. I do still think the processor adds a ton of value. We do not have many other tools for working with arrays in our documents, and this is a great tool for that.

[talevy]

@clintongormley your thoughts on this issue would be great!

[jpountz]

+1 to add the experimental annotation in case of any doubts about the stability of the API

[javanna]

but isn't ingest as a feature already experimental per se? do we want to make foreach super experimental? :)

[clintongormley]

i'm good with calling out foreach as experimental. wondering if we should change the foreach model to avoid the issues (which i don't fully understand). perhaps tomorrow we can chat on zoom?

[martijnvg]

I'll mark the foreach processor as experimental.