Allow to change FROM clause using QueryBuilder

[derevnjuk]

Description
As a user, I would like to have the ability to change/add FROM clause in which the entity's table select/update/delete will be executed.

Motivation

For example, implementing the seek pagination with the ability to navigate to the previous page, you have to reverse the ordering of the result set (ASC vs DESC) using the same predicate so that the results you want are to the right of the cursor:

SELECT * 
FROM (
  SELECT * 
  FROM users 
  WHERE createdAt < ? 
  ORDER BY createdAt DESC 
  LIMIT 10
) AS u 
ORDER BY createdAt ASC;

Currently, this is possible to do only using knex.fromRaw(), exposing the implementation details of QueryBuilder. What could be a issue from design perspective. As soon as the implementation of either QueryBuilder or knex is changed (e.g. mikro-orm starts using a different query builder from knex), the user will have to rework a significant part of his code.

Just by declaring a from() method, we are able to give the user a powerful mechanism to handle this without necessity to rely on knex:

const qb = orm.em.qb(User, 'user')
  .where({ createdAt: { $lt: new Date() } })
  .orderBy({ createdAt: 'DESC' })
  .limit(10);

const users1 = qb.from(qb.getKnexQuery())
  .orderBy({ createdAt: 'ASC' });

const users2 = qb.from(qb.clone())
  .orderBy({ createdAt: 'ASC' });

Possible solution
The new method(s) can be introduced to be used to change/add FROM clause, removing all previously set from-s:

export declare class QueryBuilder<T extends AnyEntity<T> = AnyEntity> {
    from<TFrom extends AnyEntity<TFrom> = T>(entityTarget: TFrom | Knex.QueryBuilder | QueryBuilder<TFrom>): QueryBuilder<TFrom>;
    addFrom<TFrom extends AnyEntity<TFrom> = T>(entityTarget: TFrom | Knex.QueryBuilder | QueryBuilder<TFrom>): QueryBuilder<TFrom>;
}

[B4nan]

I hope you also plan to send some PRs, and not just a huge round of issues :]

[derevnjuk]

@B4nan I hope so, but I can't do this without your help 😄