improved semantics for esaggs function

[ppisljar]

esaggs function semantics needs improvements. it should not accept any json blob parameters. Instead we should rather create utility functions.

• make esaggs accept aggs parameter which is array of agg_type
  • Add a toExpressionAst method to handle serializing an agg config into an AST
  • Create interface to serve as a mapping of all agg type params
  • Add aggTerms function as an example
  • bucket agg functions (21 agg types)
  • metric agg functions (11 agg types)
• make esaggs accept index parameter which is the return value of type index_pattern from indexPatternLoad
  • since we will already need to update downstream uses of esaggs, we can look for other expression functions that use index pattern IDs to instead take in indexPatternLoad. For items that can't be changed at once, follow up issues should be created.
• update downstream uses of esaggs (mostly build pipeline and lens)

parent issue #46902

[elasticmachine]

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