feat(core): allow declarative partial loading of collection items

[B4nan]

Collections can also represent only a subset of the target entities:

@Entity()
class Author {

  @OneToMany(() => Book, b => b.author)
  books = new Collection<Book>(this);

  @OneToMany(() => Book, b => b.author, { where: { favorite: true } })
  favoriteBooks = new Collection<Book>(this);

}

This works also for M:N relations. Note that if you want to declare more relations mapping to the same pivot table, you need to explicitly specify its name (or use the same pivot entity):

@Entity()
class Book {

  @ManyToMany(() => BookTag)
  tags = new Collection<BookTag>(this);

  @ManyToMany({
    entity: () => BookTag, 
    pivotTable: 'book_tags',
    where: { popular: true },
  })
  popularTags = new Collection<BookTag>(this);

}

Closes #4963