Tracking Issue: implement embedding mode for manifest creation

[alilleybrinker]

Right now the omnibor crate only allows manifests to be created in "non-embedding" mode, which means that when the manifest is created, the artifact itself is not updated with a reference to the manifest. This is fine and compliant with the OmniBOR spec, but we also want to support "embedding mode" where the target artifact is updated. Embedding establishes a cryptographic relationship between the manifest and the Artifact Identifier of the target, ensuring that changes in the dependencies used to build the artifact will result in a change of the identifier for the artifact itself. It's in embedding mode that OmniBOR's Artifact IDs established the Merkle-tree like quality that was part of the original value proposition for it.

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There are multiple parts to implementing this, which already have a skeleton in the existing code populated with todo!() macro calls:

• Implement target type inference for embedding mode #200
• Implement embedding for ELF files #201
• Implement embedding for text files with prefix-based comments #202
• Implement embedding for text files with wrapped comments #203

While support for a broader range of embedding contexts may be added in the future, this is at least the minimal set of embedding contexts we'd like to support at the beginning.