Do not bundle dns-plugins into frontend

[wingi11]

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

Cannot add custom made dns plugins to ngm without repackaging the whole frontend.

Describe the solution you'd like

Fetch available dns plugins from the backend and the backend responds with the dns plugins noted in certbot-dns-plugins.js

Describe alternatives you've considered

Haven't found out an alternative way to add custom dns plugins without rebuilding the whole stack.

Additional context