Add interactive standalone keystone mode

[ErikMcClure]

This still does not allow installing keystone as a service, so it must be run under a session. You can do this via cargo run via the following:

cargo run --features="binary-deps" --bin keystone -- session -i -t .\examples\example.windows.toml

the actual command here is:

keystone session -i -t .\examples\example.windows.toml

Keystone will still run without -i but it won't be interactive. -i provides the interactive TUI that lets you manage the modules you spawned.

Keystone is still using stdin/stdout for RPC communication, which will be addressed in a separate PR. This PR contains necessary refactors to make it possible to fix that.