Allow for SBOM as input

This has been touched on in separate issues, but not explicitly in it's own issue. It would be ideal to be able to take syft JSON as input to be able to translate it to another format (or another format into syft JSON). This implies having a nice way to take generically a document as input (not an image) similar to how grype does this with the `sbom:` scheme.

Start by reviewing Syft's existing input handling and Grype's `sbom:` scheme, both referenced in the issue. Determine how generic document input should support Syft JSON and conversion to or from another format; done should include an agreed input design and working format translation.

Written by the indexing model from the issue text.