Add REST API for directly downloading the latest version of an extension

[phaumer]

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At the moment afaik one needs to query the REST API for metadata and assemble the url for the latest version of an extension.

In an Eclipse Che devfile/plugin a user might want to specify to always pull just the latest version and not a specific one. A Url for directly just downloading the latest would be useful: https://open-vsx.org/api/ibm/zopeneditor/latest/file/download