feat(core): allow extending EntityManager

[B4nan]

To extend the EntityManager with your own custom methods, you can use the entityManager ORM option:

import { MikroORM, EntityManager } from '@mikro-orm/sqlite';

class MyEntityManager extends EntityManager {

  myCustomMethod(base: number): number {
    return base * Math.random();
  }

}

const orm = await MikroORM.init({
  entities: [...],
  dbName: ':memory:',
  entityManager: MyEntityManager,
});
console.log(orm.em instanceof MyEntityManager); // true
const res = orm.em.myCustomMethod(123);

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (66a6b75) 99.79% compared to head (3038a9f) 99.79%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5064   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files         221      221           
  Lines       16383    16386    +3     
  Branches     3954     3954           
=======================================
+ Hits        16349    16352    +3     
  Misses         34       34           

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