Add support for "db types"

[Pierrci]

This PR adds the ability to retrieve the "types" associated with the collections of a db from a remote service (one URL per server), allowing to link between documents from different collections.

See

Mongoku/lib/Database.ts

Lines 17 to 32 in 25492ac

	export type Reference = {
	/** The external collection this references links to */
	collection: string;
	/** The key in the external collection, useful to generate a query based on it */
	key: string;
	}
	/**
	* List of keys in the collection documents that are associated with documents in other collections
	* The collection a key refers to can vary from one document to another, hence the array
	*/
	export type CollectionRefs = { [prop: string]: Reference[]; }
	export type DbRefs = {
	[collection: string]: CollectionRefs;
	}

[julien-c]

maybe this could be controlled by an env var?

[Pierrci]

it's not mandatory, but yes why not

[julien-c]

(im wondering if some other people are using Mongoku in prod btw)

[Pierrci]

it can be some "experimental" feature flag maybe

[Pierrci]

mmm not sure it's worth the hassle for one feature, this is old technology we're working with here - no simple way to pass the value of a server-side env var to the front