Fork Expressions service

[streamich]

Add ability to expressions plugin to fork Expressions service, that will allow Canvas and Lens to have their private instances of Expressions service. Their private instances would have access to all global expression types and expression functions, but they could also register local expression types and expression functions that would be available only in local forks of Expression service.

Needs discussion: How conflicts are resolved, e.g if a function with the same name is registered in global and forked expression service?

• Add fork() method to expression plugin's setup contract.
• Forked instance of Expressions service should have access to all newly registered expression types and functions from global instance.
  • Expression renderers can be shared?
• Forked instance should allow to register expression types and functions that are only visible in local fork.
• Forked instance should have all the same functionality as global instance.

Parent issue: #46909

[elasticmachine]

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