[5.x] Add `get_site` tag

[aerni]

This PR takes another stab at https://github.com/statamic/cms/pull/9567 by introducing a new get_site tag.

Use as tag pair:

{{ get_site:english }}
    {{ permalink }}
{{ /get_site:english }}

You can also get the site dynamically:

{{ get_site handle="{handle}" }}
    {{ permalink }}
{{ /get_site }}

Or use as single tag:

{{ get_site:english:permalink }}