StructBuilder: support with_annotations

[mtcalvin]

I'm using StructBuilder to dynamically construct/modify Ion structs. In addition to setting struct fields, I also need to add annotations to the struct.

Currently, I have to .build() the builder and then call .with_annotations([...]) on the result. It would be nice for API symmetry if annotations could be added/removed as part of the builder (like fields) instead of only on the resulting object.