Web API: make /origin/ return the swh:1:ori:... PID

it currently return something like this:

{
    "id": 1,
    "origin_visits_url": "/api/1/origin/https://github.com/hylang/hy/visits/",
    "type": "git",
    "url": "https://github.com/hylang/hy"
}

I propose to extend it like this:

{
    "id": 1,
    "ori_id": "swh:1:ori:7bed762bf5ddc7164a08dac613ead782784474bb",
    "origin_visits_url": "/api/1/origin/https://github.com/hylang/hy/visits/",
    "type": "git",
    "url": "https://github.com/hylang/hy"
}

The id v. ori_id isn't great, but it seems we are already inconsistent in how we call the ID field in other methods (e.g., revision uses id while directory uses dir_id (for subdirs)). ori_id seems the best way out of this.

@vlorentz: can you note down as additional TODO item for API v2 to consistently use pid as field name for, well, PIDs?

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Migrated from T1970 (view on Phabricator)