Implement embedding for ELF files

[alilleybrinker]

One of the types of targets we want to support embedding for is ELF files. The OmniBOR specification includes specific descriptions of how information should be embedded in a custom notes section, so we'll defer to that for instructions. The key thing here will be to leverage existing Rust libraries for interacting with ELF files. I know the Gimli project maintains the object crate, though I will admit I am unfamiliar with this area generally and so can't give strong guidance on how to go about implementing this embedding.