start working on composition mode

[mandelsoft]

Description

THis PR starts the introduction of the composition mode.
So far, a component version is always directly connected to the stored version on a backing storage.
Closing a component version updated the changes directly to this medium.

THis will be changed in the future. While still connecting to a backing storage updates will
only be done, if explicitly requested, either by

• calling Update on the component version
• or by adding the version to a component using AddVersion

The new behaviour is already available by a switch, while the standard behaviour is still the default.

..../ocm/pkg/contexts/ocm/attr/compositionmodeattr.Set(true)

Additionally versions in general, and versions created via NewVersion can be used to call AddVersion on all repositories and not only at the component object, they were created for.

A new repository type composition is used to handle pure Go-object based OCM repositories. Even free-floating component version can now be created and composed (based on this repository type) and then added to any other repository.
If used with the composition repository type a complete set of component version can be prepared in memory and then transferred to any other repository using the transfer package. Basically the composition repository behaves like any other repository.

THis could be achieved by extending the blobs to support the Dup functionality and feature a ref-counting. This way
blobs are stored in the composition repository keeping large blob content potentially held in temporary filesystem resources until the last reference is closed. THis optimized the memory footprint, when using large blobs, because even for using local blobs in the repo, the content is always taken from the original blob access,

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation Update
• 🎨 Style
• 🧑‍💻 Code Refactor
• 🔥 Performance Improvements
• ✅ Test
• 🤖 Build
• 🔁 CI
• 📦 Chore (Release)
• ⏩ Revert

Related Tickets & Documents

• Related Issue # (issue)
• Closes # (issue)
• Fixes # (issue)

Remove if not applicable

Screenshots

Added tests?

• 👍 yes
• 🙅 no, because they aren't needed
• 🙋 no, because I need help
• Separate ticket for tests # (issue/pr)

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Added to documentation?

• 📜 README.md
• 🙅 no documentation needed

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules