Expose the `InputManifestBuilder` API over FFI

[alilleybrinker]

The omnibor crate includes an FFI module to enable reuse of this Rust implementation by other languages. Today, that FFI module only includes support for working with ArtifactIds, and has nothing for building new manifests. Support for the InputManifestBuilder API implies support for working with Storage types and with the InputManifest itself.

This is likely to be a complex issue. If anyone wants to tackle it, I highly recommend talking with us here before picking it up so we can mentor on how to achieve it.

[alilleybrinker]

At this point I think the input manifest builder API is probably stable enough that it makes sense to pursue this.