Allow retrieving localized assets

[florian-besser]

This should fix #45 and fix #60

By specifying the target locale of an asset to retrieve we can correctly handle the data given by contentful.

Sample contentful data used to test this PR with:

{
  "metadata": {
    "tags": []
  },
  "sys": {
    "space": {
      "sys": {
        "type": "Link",
        "linkType": "Space",
        "id": "censored"
      }
    },
    "id": "censored",
    "type": "Asset",
    "createdAt": "2021-03-02T11:25:00.594Z",
    "updatedAt": "2021-03-02T11:28:53.219Z",
    "environment": {
      "sys": {
        "id": "master",
        "type": "Link",
        "linkType": "Environment"
      }
    },
    "publishedVersion": 2,
    "publishedAt": "2021-03-02T11:28:53.219Z",
    "firstPublishedAt": "2021-03-02T11:28:53.219Z",
    "createdBy": {
      "sys": {
        "type": "Link",
        "linkType": "User",
        "id": "censored"
      }
    },
    "updatedBy": {
      "sys": {
        "type": "Link",
        "linkType": "User",
        "id": "censored"
      }
    },
    "publishedCounter": 1,
    "version": 3,
    "publishedBy": {
      "sys": {
        "type": "Link",
        "linkType": "User",
        "id": "censored"
      }
    }
  },
  "fields": {
    "title": {
      "id-ID": "Article-12.2 01"
    },
    "file": {
      "id-ID": {
        "url": "//images.ctfassets.net/censored/censored/censored/Article-12.2_01.jpg",
        "details": {
          "size": 415255,
          "image": {
            "width": 1080,
            "height": 1920
          }
        },
        "fileName": "Article-12.2_01.jpg",
        "contentType": "image/jpeg"
      }
    }
  }
}