api: Query parameters serializers

endpoints

• citation
• content
• directory (no query params)
• extid
• graph (no query params)
• identifiers (no query params)
• metadata
• origin
• ping (no query params)
• raw (no query params)
• release (no query params)
• revision
• snapshot (already done in !1367 (merged))
• stat (no query params)

other changes

To validate origins the package rfc3987 has been added to the dependencies.