Make return types consistent: use Option<&str> instead of &Option<String>

[ysaito1001]

For struct getters that return &Option<String> throughout the codebase, replace them with Option<&str>.

There are structs in the AWS SDK crates that return &Option<string> but this is due to the smithy-rs code generator mechanically determining the return type as &Option<T> given a struct field of type T. If we can afford to handwrite the Rust code, we should prefer more idiomatic Option<&str>.