maven: Pass the base URL of the Maven instance to the loader

I would like to use it as the metadata authority URI in the loader,
instead of '{p_url.scheme}://{p_url.netloc}/', which I do not think
is accurate, as it is possible to have multiple Maven instances at
the same netloc.

swh/lister/maven/lister.py

@@ -302,6 +302,7 @@ class MavenLister(Lister[MavenListerState, RepoPage]):
                             "gid": page["gid"],
                             "aid": page["aid"],
                             "version": page["version"],
+                            "base_url": self.BASE_URL,
                         }
                     ]
                 },

swh/lister/maven/tests/test_lister.py

@@ -122,6 +122,7 @@ def test_maven_full_listing(
                     assert src.get("gid") == artifact["gid"]
                     assert src.get("aid") == artifact["aid"]
                     assert src.get("version") == artifact["version"]
+                    assert MVN_URL == artifact["base_url"]
                     break
             else:
                 raise AssertionError