Allow adding custom metadata to exports

[vycius]

When exporting repositories from one Pulp instance and importing them on another, multiple exports may arrive simultaneously. It is currently difficult to identify key details (e.g., repository name, export type: full vs. incremental) without inspecting large tar files. This slows down automation and makes import order logic harder.

Closes #6712

Solution

This change introduces a new optional meta field to the export serializer.

• Accepts arbitrary key-value metadata at export time.
• Metadata is written into the toc.json file under the meta section.
• The same metadata is also stored in toc_info for quick lookup.

This allows exporters/importers to embed identifying information (e.g., repo name, export type, author) directly in the TOC, making exports easier to classify and manage.

Example

Export request

client.post(
    f"{exporter_href}exports/",
    json={
        "full": full_export,
        "chunk_size": chunk_size,
        "meta": {
            "repositories": ["example/repository"],
            "author": "pulp"        
        },
    },
)

TOC json

{
  "meta": {
    "repositories": ["example/repository"],
    "author": "pulp",
    "checksum_type": "crc32",
    "chunk_size": 53687091200
  },
  "files": {
    "export-019a01ce-20251020_1328.tar.0000": "96903fec"
  }
}

Exporter list output

{
  "params": {
    "full": true,
    "meta": {
      "repositories": [
        "example/repository"
      ],
      "author": "pulp"
    },
  },
  "toc_info": {
    "file": "/export-019a01d7-20251020_1338-toc.json",
    "meta": {
      "repositories": [
        "example/repository"
      ],
      "author": "pulp"
    },
    "sha256": "49c9ea862f67efc04c4"
  }
  ...
}

[dralley]

@vycius Could you please rebase on top of the current state of the main branch?

[vycius]

@vycius Could you please rebase on top of the current state of the main branch?

Rebased.

[ggainey]

This looks reasonable, but def needs tests. You can add here https://github.com/pulp/pulpcore/blob/main/pulp_file/tests/functional/api/test_pulp_export.py

[vycius]

This looks reasonable, but def needs tests. You can add here https://github.com/pulp/pulpcore/blob/main/pulp_file/tests/functional/api/test_pulp_export.py

Tests added