remote-data-example extension

[matthewhanson]

Create example extension remote-data, and publish it. The examples in https://github.com/radiantearth/stac-spec will use it to validate.

[matthewhanson]

@cholmes @m-mohr want to review this example extension: https://github.com/stac-extensions/remote-data

If looks good we can add it to stac_extensions of the example in stac-spec