Rust: (for 1.0.0) change lookup function return signature from Result<String, VaultError>->Result<Vec<u8>, VaultError> or similar

[kahlstrm]

If we look at the current Vault::store function signature:

 pub async fn store(&self, name: &str, data: &[u8]) -> Result<(), VaultError>

compared to Vault::lookup:

 pub async fn lookup(&self, name: &str) -> Result<String, VaultError>

There is immediately the problem where we store something else than a valid UTF-8 string and try to look that up.

As we support storing encrypted arbitrary files, we should support looking up something that is not a valid UTF-8 string

This change breaks the current public interface, so I propose that this should be in the major 1.0.0 release.