feat: create spinel connection module

[jdswensen]

Create a module with a trait that is capable of abstracting away the framing details from the host caller. Something similar to:

pub trait SpinelHostConnection {
    async fn noop(&self) -> Result<(), Error>;
    async fn reset(&self) -> Result<(), Error>;
    async fn last_status(&self) -> Result<(), Error>;
}

This will allow for targeting different platforms (posix vs dual-core MCU).