Allow sidebar links to customise target & rel attributes

[g-dx]

Is your feature request related to a problem? Please describe.

Now that the pathname protocol has been removed (5836fac) I believe there is no way to configure a sidebar link which points to an external resource.

Describe the solution you'd like

I assume we need the same customisation as is available for nav items:

https://vitepress.dev/reference/default-theme-nav#customize-link-s-target-and-rel-attributes

Describe alternatives you've considered

No response

Additional context

No response

Validations

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