feat: optimize build

stellar-cli v27 includes `--optimize` by default.

We just need to include `additional-libs` in order to enable it.

This makes the build for `stellar-scaffold-cli` much bigger!

Alternatives
------------

Rather than embedding `stellar-cli`, we could shell out to it. This
would require a more significant re-architecture. Is it worth it, before
shipping this feature? (Ideally, we should do some more data-driven
comparison of the current approach vs status quo to understand the
increased build/install time of `stellar-scaffold-cli` to see if we
think it's worth going with the simpler-to-ship approach for now.)

Author: chadoh