[Expressions] expressions setup/start contract leak wrong functions

[ppisljar]

expressions setup and start contract expose functions they shouldn't:

• setup contract exposes run which should only be available in start when all the plugins had a chance to register their functions
• start contract exposes fork which should only be available in setup so function registration can happen in time

on top of that forked expression service exposes all the methods at all times (so addFunction, addType etc can be called in start or after and run and execute can be called in setup) this should be cleaned up so fork is only available in setup, it returns setup contract of fork and a function to call to get start contract of fork, which should have a guard against using it before start.

we should also add guards in addFunction and addType methods to prevent them from being called after setup (even if consumer ignores the types)

note: some consumers of expressions are already registering functions at wrong times, we should investigate if that can be easily changed.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)