support the `alternate-assets` extension

[keewis]

With the alternate-assets extension it is possible to specify different access methods:

{
  "assets": {
    "product": {
      "href": "https://...",
      "type": "...",
      "alternate:name": "https",
      "alternate": {
        "s3": {
          "href": "s3://...",
          "storage:refs": "aws-s3",
          "alternate:name": "s3"
        }
      }
    }
  }
}

xpystac should allow selecting one of the alternate access methods (defaulting to the href declared on the asset itself)