Support Mirroring a Composer v2 Registry

[Kenneth-Sills]

Problem

Per "Supported Registry Formats", the only supported mirrors are those using Composer v1 metadata formats. This restricts the mirroring feature to Satis, Packeton, and Private Packagist. However, the latter is planning on dropping support for Composer v1 formats at the end of this year.

That means, it's not possible to mirror a Pricore registry from another Pricore registry currently, or to mirror the official Packagist registry.

Proposed Solution

We should implement Composer v2 support and, when setting up a mirrored registry, the latest supported protocol should be used.

Alternatives Considered

No response

Additional Context

No response

[maartenbode]

Opened #151 to add this. It introduces a v2 registry client that reads the Composer v2 metadata-url layout, so you can now mirror another Pricore instance or a v2-only registry like Private Packagist. The factory detects and prioritizes v2, with the existing v1 inline/includes formats kept as fallbacks.

Mirroring full Packagist is out of scope — I don't think eagerly mirroring ~400k packages makes sense. Registries that publish an available-packages list work as expected.