Database eager relations

When using db.get provided by @dzlzv/hydra-common's DatabaseManager like this:

// input schema
type Video @entity {
  channel: Channel
  license: License
  ...
}

// db.get in mappings
const video = await db.get(Video, { where: { id: videoId.toString() } as FindConditions<Video> })

a flat video record without related objects loaded is returned

Video {
  channelId: '3',
  licenseId: '1',
  ...
}

// instead of what I expected
Video {
    channel: Channel { ... },
    license: License { ... },
}

There is a way to load related object by enumareting them in db.get parameters. That works, but
a) only 1 level deep e.g. I can access video.channel, but can't access video.channel.category.name.
b) brings extra complexity to mappings
c) requires keeping list of relations in mappings
See https://github.com/typeorm/typeorm/blob/51b2a63d/src/find-options/FindOneOptions.ts#L23

const video = await db.get(Video, { where: { id: videoId.toString() } as FindConditions<Video>, relations: ['channel', 'license'] })

From what I found there is a way to automatically load related objects if we expose typeorm's find* functions (preferebly) or QueryBuilder in DatabaseManager.
See https://orkhan.gitbook.io/typeorm/docs/eager-and-lazy-relations

If we introduce find* functions, https://github.com/Joystream/hydra/issues/370 will be solved as well.

Start at the @dzlzv/hydra-common DatabaseManager and its db.get entry point, then compare its options with TypeORM FindOneOptions and the eager and lazy relations guidance. Determine whether exposing find* functions or QueryBuilder can load nested related objects without mapping relation lists, and verify that the approach also addresses issue 370.