add feature surf in sophia to enable surf_loader in sophia_jsonld

more generally, this feature in sophia is aimed to enable all features
depending on `surf`.
For example, I plan to add a SurfLoader to sophia_resource eventually,
which should also be enabled by `sophia/surf`.

sophia/Cargo.toml

@@ -21,6 +21,8 @@ jsonld = ["sophia_jsonld"]
 xml = ["sophia_xml"]
 # This feature enables to use the graph and dataset test macros in other crates
 test_macro = ["sophia_api/test_macro"]
+# This feature enables the use of the surf crate in dependencies
+surf = ["sophia_jsonld/surf_loader"]
 
 [dependencies]
 sophia_iri.workspace = true