Implement an OCI garbage collector

[LucaSeri]

Feature request summary

OCI image packages are stored as layers in the kraftkit runtime directory.

Due to how images are constructed, a layer might be used in more than one package. E.g. two packages might be using the same kernel with a different rootfs. To optimize this, the kernel is not packaged twice, it instead lives in its own layer that is only referenced by the two packages.

The issue occurs when we want to remove a package from our OCI store, as there is currently no reference counting mechanism. This means that we have to pick between two not so great options:

• remove all the dependencies, potentially breaking other packages
• remove no dependencies, causing the cache to grow indefinitely (albeit, slowly)

This issue tracks implementing a better garbage collection mechanism to only remove the necessary dependencies.

This involves changes to the DeleteIndex method of the different OCI handlers, e.g.:
https://github.com/unikraft/kraftkit/blob/staging/oci/handler/directory.go#L1087

Describe alternatives

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Related architectures

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Related platforms

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Additional context

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