LB3 model import: handle ObjectID type for MongoDB

[bajtos]

In LB3, the type of the primary key is determined by the connector. The MongoDB connector defines the type as ObjectID, using a class provided by MongoDB (bson module).

Conceptually:

{
  "name": "Product",
  "properties": {
    "id": {
      "id": true,
       "generated": true,
       "type": ObjectID
    }
  }
}

In LB4, we treat ObjectID values differently:

• the property type is set to string
• there is MongoDB-specific metadata to tell the connector to use ObjectID data-type for this property

@property({
  type: 'string',
  id: true,
  generated: true,
  mongodb: {dataType: 'ObjectID'},
})
id: string;

We need to improve the importer to detect ObjectID properties and convert them to LB4 format.

This task depends on #3810 LB3 model import: preserve connector-specific metadata in property definitions, because we need to define connector-specific metadata.

Acceptance criteria

• Implement conversion of primary key (id) properties
• Check how are foreign keys handled, implement conversion for them too if needed
• Consider converting all properties with ObjectID type, regardless of whether they are PK/FK or not
• if we see the property as ObjectID, we convert it to the appropriate mapping.
• Update Importing-LB3-models.md, remove the relevant point from the list of known limitations

[dhmlau]

Moving to 2020Q1 as the blocking items are not target for Q4.

[stale]

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[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.