Allow embedded runtime to be fully disabled

[josevalim]

For security concerns. Disabling should also be available via an environment variable.

[jonatanklosko]

Another option would be to have configuration for listing what runtime types should be available. By default we could even omit embedded altogether.

[josevalim]

I think omitting embedded by default is good and then just a flag to turn it on. :)

[ByeongUkChoi]

Do we need these environment variable like this LIVEBOOK_ENABLE_EMBEDDED_RUNTIME ​​here?
then, what if LIVEBOOK_DEFAULT_RUNTIME=embedded and LIVEBOOK_ENABLE_EMBEDDED_RUNTIME=false?

[josevalim]

If it is disabled by default, then we don't need an env var, because the only use case we have so far is for Nerves and Nerves has its own config. So we just need something like config :livebook, :enable_embedded_runtime, true. :)

[jonatanklosko]

@josevalim for nerves both Elixir and Mix standalone aren't useful, maybe instead we should allow config :livebook, :runtime_modules, [Livebook.Runtime.Embedded, Livebook.Runtime.Attached] ?

[josevalim]

I like that as long as embedded is disabled by default! @ByeongUkChoi, Can you please revisit your PR?

[ByeongUkChoi]

that means, make config :livebook, :runtime_modules and check runtime module when select runtime type and connect runtime?

[josevalim]

Yes!

[jonatanklosko]

We can also dynamically for on the list of modules, so we match the configured order as well :)

[jonatanklosko]

Closed via #1084.