Add Nextclade extension

[j23414]

Context

Add the Nextclade extension to auspice_config.json to enable "View in other platforms"

Possible solution

• Probably requires a similar fix to Add Nextclade extension to public builds seasonal-flu#197

[jameshadfield]

Add the Nextclade extension to auspice_config.json to enable "View in other platforms"

It doesn't need any extension it just needs a root-sequence (either in-line or via sidecar JSON)