Make subkey transfer feature optional or disableable via feature flag

[xlc]

I need to be able to do sr25519 in python/go/any non rust non js environment but there is no sr25519 library available AFAIK. So the quick workaround is invoke subkey via command line.

I want an executable that is small and lean but subkey currently depends on node-runtime which bundles the whole substrate runtime, which is not useful for my case and useless for any other non substrate chain.

It also make build slow.

Transfer feature can be enabled by default as long as there is a way to disable it during the build.

[burdges]

We need someone to write C bindings for schnorrkel eventually. See w3f/schnorrkel#32

[folsen]

I think it makes sense to just build wrapper libraries around the rust sr25519 libraries here rather than depend on subkey at all, I don't think we should all be building all of our infrastructure around subkey because there is no native language support. The benefit of Rust is that you can generate C-bindings really easily, then use them in Python/Go/whatever.

[burdges]

Someone did a partial one https://gitlab.com/kauriid/schnorrpy/