Update the Client interface to allow push operations to:

• Prepare a "staging area" for the full SBOM external reference graph
• Recursively add all externally referenced SBOMs to this staging area
• Do a single push operation containing all staged SBOMs

Example usage:

• local Git worktree: after cloning, resolve and stage all external references, then single commit and push to remote
• local OCI store: create local OCI storage in memory, create a manifest descriptor, add all SBOMs as artifacts to the manifest, copy to memory storage, push memory storage to remote