Make metadata_type_values to Optional for MetaData

Nayeem Rezarequested to merge
fix/metadata-pydantic into master

In dma.test get-metadata gives back this response

[
  {
    "id": 288075,
    "dataId": 300535,
    "metadataType": {
      "id": 21,
      "name": "STOCKS_METADATA",
      "format": "JSON"
    },
    "value": "{\"dataOwner\":\"girardot\"}"
  }
]

which doesn't fit with the pydantic model of MetaData

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