feat(core): add support for nested partial loading

[B4nan]

Adds support for nested partial loading:

const author = await orm.em.findOne(Author, '...', { fields: ['name', 'books.title', 'books.author', 'books.price'] });

Or with alternative object syntax:

const author = await orm.em.findOne(Author, '...', { fields: ['name', { books: ['title', 'author', 'price'] }] });

It is also possible to use multiple levels:

const author = await orm.em.findOne(Author, '...', { fields: ['name', { books: ['title', 'price', 'author', { author: ['email'] }] }] });

Primary keys are always selected even if you omit them. On the other hand, you are responsible
for selecting the FKs - if you omit such property, the relation might not be loaded properly.
In the following example the books would not be linked the author, because we did not specify
the books.author field to be loaded.

// this will load both author and book entities, but they won't be connected due to the missing FK in select
const author = await orm.em.findOne(Author, '...', { fields: ['name', { books: ['title', 'price'] });

Same problem can occur in mongo with M:N collections - those are stored as array property
on the owning entity, so you need to make sure to mark such properties too.

Available for testing since 4.3.5-dev.77.

Closes #221

[astahmer]

That's a great addition ! Thank you for this.