Expressions: move esaggs to the server

[ppisljar]

In order to support alerting or any other server side solution that would want to run expressions we need to move esaggs function to the server.

• improve semantics of esaggs function: improved semantics for esaggs function #61768
• make index patterns service available on the server & add indexPatternLoad expression function [Meta] Index Patterns #67574 indexPattern expression function #62106
• make search service fully available on the server [Search] Expose same API on client/server #55583
• make aggs available on the server [data.search.aggs] Make aggs available on the server #65793
• make search source available on the server [data.search.searchSource]: Expose SearchSource on server #65069
• move esaggs function to the server
  • unit tests for esaggs
  • split courier request handler out to common directory
  • slightly different function definitions on client & server due to differences in dependent services

parent issue: #57344

[elasticmachine]

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