LB3 model import: preserve connector-specific metadata in property definitions

[bajtos]

Consider the following LB3 model definition:

{
  "name": "Product",
  "base": "PersistedModel",
  "properties": {
    "name": {
      "type": "string",
      "required": true,
      "mysql": {
        "column": "NAME",
        "length": 80,
      }
    }
  }
}

Expected property definition in LB4 model file:

@property{
  type: 'string',
    mysql: {
      column: 'NAME',
      length: 80,
    },
})
name: string;

• UPDATED Acceptance criteria (02/24/2020) based on LB3 model import: preserve connector-specific metadata in property definitions #3810 (comment)

Acceptance criteria

• add migration unit tests for preserving db metadata in model properties
• check a migrated lb-next app manually that was migrated from an lb3 app with the above criteria
• open additional issues for any open challenges migrating apps with above criteria
• Update Importing-LB3-models.md, remove the relevant point from the list of known limitations
• remove blocks on dependent issues (LB3 model import: handle ObjectID type for MongoDB #3814)

[deepakrkris]

@bajtos I find that lb4 import-lb3-models command already supports including connector metadata in property definitions (I checked out of curiosity because the existing code does not restrict flowing of db metadata from lb3)