Add validation for absolute self link in item schema (#1281)

* Add validation for absolute self link in item schema

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - The `keywords` field known from Collections is available in common metadata. ([#1187](https://github.com/radiantearth/stac-spec/issues/1187))
 - The `license` field additionally supports SPDX expressions and the value `other`.
 - The `roles` field known from Assets and Providers is available in common metadata. ([#1267](https://github.com/radiantearth/stac-spec/issues/1267))
+- Validation for absolute self link in item schema. ([#1281](https://github.com/radiantearth/stac-spec/issues/1281))
 - Best practice: Link titles should exactly reflect the title of the corresponding entity ([#1168](https://github.com/radiantearth/stac-spec/issues/1168))
 
 ### Changed

item-spec/json-schema/item.json

@@ -204,6 +204,21 @@
               "title": "Link title",
               "type": "string"
             }
+          },
+          "$comment": "Link with relationship `self` must be absolute URI",
+          "if": {
+            "properties": {
+              "rel": {
+                "const": "self"
+              }
+            }
+          },
+          "then": {
+            "properties": {
+              "href": {
+                "format": "iri"
+              }
+            }
           }
         },
         {