OCI: use oras/v2

[srenatus]

ORAS is doing a revamp in their v2 work: among other things, this should cut down dependencies significantly, and that would be very welcome to the use in OPA.

• See oras.land/oras-go/v2 and its directories for the new interface.
• The Copy API we're currently using is already ported.
• The target of the copy seems to allow for different things:
  • file
  • memory memory could be interesting for us when there is not persistence directory.

I think this issue should perhaps precede the other OCI issues (#4637 and #4639 and #4638) since it has consequences on their content or implementation. (WDYT @carabasdaniel?)

[carabasdaniel]

Hi @srenatus, I think we might be able to tackle the bug first and then focus on bumping to oras/v2 before any other feature work.

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[srenatus]

https://github.com/oras-project/oras-go/issues/49 done, alpha release out.

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[anderseknert]

This was resolved in https://github.com/open-policy-agent/opa/pull/5690 Thanks @ashutosh-narkar 👍