Implement extrinsic origin metadata specification.

#2075 (closed)

Depends on !385 (closed).

---

Migrated from D2988 (view on Phabricator)

docs/extrinsic-metadata-specification.rst

@@ -32,11 +32,11 @@ Metadata authorities are entities that provide metadata about an
 An authority is uniquely defined by these properties:
 
   * its type, representing the kind of authority, which is one of these values:
-  * `deposit`, for metadata pushed to Software Heritage at the same time
-    as a software artifact
-  * `forge`, for metadata pulled from the same source as the one hosting
-    the software artifacts (which includes package managers)
-  * `registry`, for metadata pulled from a third-party
+    * `deposit`, for metadata pushed to Software Heritage at the same time
+      as a software artifact
+    * `forge`, for metadata pulled from the same source as the one hosting
+      the software artifacts (which includes package managers)
+    * `registry`, for metadata pulled from a third-party
   * its URL, which unambiguously identifies an instance of the authority type.
 
 Examples:
@@ -145,6 +145,7 @@ The storage API offers three endpoints to manipulate origin metadata:
   added from this origin, in the format::
 
       {
+        'origin_url': ...,
         'authority': {'type': ..., 'url': ...},
         'fetcher': {'name': ..., 'version': ...},
         'discovery_date': ...,