Allow opening ds-metadata-uri-values links in a new window

[alanorth]

Is your feature request related to a problem? Please describe. It would be nice if we could indicate that metadata using the <ds-metadata-uri-values> selector on the simple item page could be opened in a new window (ie target=_blank in HTML). The rationale is that, if the user clicks a link that leads away from our site, we want it to open in a new window.

Describe the solution you'd like We should add a new optional @Input to the <ds-metadata-uri-values> selector to allow specifying the link target. This could be passed to the component by other components, for example the <ds-item-page-uri-field> selector.