Is your feature request related to a problem? Please describe.
I think it would be useful to be able to add filters to the join conditions.

Describe the solution you'd like

@Entity()
export class Post {
  @OneToMany(() => Comment, comment => comment.post)
  @Filter({ name: "enabledComments", cond: { enabledComments: true } })
  @FilterJoinTable({ name: "notDeletedComments", cond: { isDeleted: false } })
  comments = new Collection<Comment>(this);
}

const posts = await orm.em.find(Post, { title: "Example" }, { 
  populate: ["comments"]
});

It should generate a query like this:

select * from "Post" as p
  join "Comment" as c on (p.id = c."postId")
    and (p."enabledComments" = true)
    and (c."isDeleted" = false);

See Hibernate docs (Example 19.2. Using @FilterJoinTable for filterting on the association table)

Describe alternatives you've considered
I've read about filters and LoadStrategy.SELECT_IN, but I think LoadStrategy.SELECT_IN is bad for performance