Make Plugin entities accept a function

[michaelbromley]

Is your feature request related to a problem? Please describe.
When defining a plugin, the entities metadata currently expects an array of TypeORM entities. Sometimes a plugin might need to use dynamic logic to determine which entities to define.

Describe the solution you'd like
Change the metadata signature:

- entities?: Array<Type<any>>;
+ entities?: Array<Type<any>> | () => Array<Type<any>>;

Additional context
This is needed for a plugin I'm working on which can be configured by the user to store analytics either in the DB (requiring entities to be defined) or in an external data store.