feat(layouts): allow consuming a layout from a url

[imsnif]

This adds the ability to consume layouts with an http:// or https:// prefix anywhere we consume layouts.

So for example:
zellij --layout https://example.com/my-layout.kdl will start a new session with the fetched layout
zellij action new-tab --layout https://example.com/my-layout.kdl will open new tab(s) from the specified layout

This also works for plugins with the existing switch_session_with_layout and new_tabs_with_layout commands.