indexPattern expression function

[ppisljar]

To make reference extraction from expressions easy/possible we should limit the usage of core api's to specific functions. The only function loading the index patterns should be indexPattern function, so we know to search for it and can extract the index pattern id and replace it with reference.

• create utility methods to serialize/deserialize index pattern instance
• create expressions index pattern type (which is the type this function will return)
• create index_pattern expression function that takes id as an argument and returns serialized version of index pattern

followup:

• update any functions currently accepting index pattern id
  • ensure args are requiring the new index pattern type
  • esaggs will need to be updated; this will be covered in improved semantics for esaggs function #61768

related: #59960
blocks: #61768
parent issue: #61769

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[lukeelmers]

I'm going to close this issue now that we've introduced indexPatternLoad. i will plan to address the follow up items in #61768