the unique identifier of an extension. The service looks for
an installed package with the given id and returns the URL to
the root of the package. If the service can not find a matching
package, an emty string will be returned.
check if there are updates available for an extention.
Parameter extensionId
the unique identifier of an extension. When the extensionId is empty,
the service looks checks all installed extensions for a newer version,
otherwise the service looks for updates for an installed package with
the given id.
If the service finds an update for an extension, a sequence containing the name
and the version will be returned for each extension which has an update.
If the service can not find a matching package, an empty sequence is returned.
