feat: simplify how subjects are created from existing manifest

[tumido]

Hi folks 👋

I've noticed the Python Oras client library has kind of rudimentary support for attached artifacts. I'd like to address a couple of UX caveats in several PRs. Please let me know if it's heading in the right direction or if you have a different perspective.

This PR addresses a difficulty when creating a Subject reference.

As of today, an oras-py user needs to know how the subject spec is constructed, and how to calculate size and digest. This makes it quite hard to come up with a usable structure that can be used when pushing an artifact. I've addressed this by adding a from_manifest class method to the Subject data class. There may be a nicer, cleaner solution than a class method, let me know what you think.

[vsoch]

I like how this works! The reason the support is simple is because the interaction is more complex than seems at face value. If we want to fully support a client doing this interaction, the full thing is described here: https://github.com/opencontainers/distribution-spec/blob/main/spec.md#pushing-manifests-with-subject Let me know your thoughts.

[tumido]

If we want to fully support a client doing this interaction, the full thing is described here: opencontainers/distribution-spec@main/spec.md#pushing-manifests-with-subject Let me know your thoughts.

Well... I kinda don't like when things are half-done. 🙂 So.. I'll look into it and try to improve the push logic. (I may not have much time in the next two weeks though...)

Do you think it would make sense to split the push logic into push and attach methods, similarly to what oras CLI does? https://oras.land/docs/commands/oras_attach

[vsoch]

Do you think it would make sense to split the push logic into push and attach methods, similarly to what oras CLI does? https://oras.land/docs/commands/oras_attach

Since we don't have a command line client, let's start with what you have now - a push that takes a subject. If "attach" comes down to just doing that command again, we don't need attach. But if there is another use case not covered, we can consider adding it.

[tumido]

@vsoch does anything have to change in this PR or is this fine? 🙂

[vsoch]

You'll want to finish up by:

• Bump the version in oras/version.py
• Adding a line to the CHANGELOG.md with the change and new version

[tumido]

Done, thank you. 🙂