feat: Support "type" key in sub-properties in 3.x

[hacksparrow]

Description

Currently, a model definition like this:

  "properties": {
    "items": {
      "type" : {
        "itemname": { "type": "string"},
        "type": { "type": "string" }
      }
    }
  }

results in invalid data.

{ items: [object Object] }

It is caused by the sub-property type conflicting with the type resolution object.

This PR, allows sub-property type to be used without any problem.

Related issues

strongloop/loopback#4131

Checklist

• New tests added or existing tests modified to cover all changes
• Code conforms with the style
  guide

[bajtos]

Is this a back-port of #1693? Was the back-port trivial, or did you have to make any significant changes?

[dhmlau]

Seems to be a backport.. the changes are the same as #1693.

[hacksparrow]

@bajtos it was a trivial backport.