Adapt the footer function to also take external links

[cjrace]

Describe the solution you'd like

We found an example of wanting an external link in the footer as well as internal links, in dfe-analytical-services/shiny-template#120 (review)

Ideally we'd allow external links as well as internal links, and where they're used they make use of our external_link() function, but with the opens in new tab warning there for screen readers only (as I've not seen any GOV site have opens in new tab in a footer link, so I'm assuming it's expected behaviour that it changes the page given the link context in the footer).

Describe alternatives you've considered

Not considered any other alternatives, assuming we can find a way to pass in both external and internal links, even if we move to accepting a dataframe as the input so you can customise the link text / link url / whether it is external

Additional context

Example GOV.UK site with a mix of links in the footer - https://explore-education-statistics.service.gov.uk/