Allow native Runtime execution

Since !229 (merged), the --execution native CLI option is not effective anymore. This was planned both by Polkadot and our team.

However, I feel a bit lost when I cannot put Breakpoints in the Runtime while developing: I use theme a lot both for understanding Runtime behavior and global Client code call stack. It is very effective for my global comprehension of Substrate code.

I understand why we removed this option. I suggest that we bring it back in another form: a Cargo feature flag allowing for its usage by developers:

cargo run --feature native