new datatype segmentidentity to represent segment id

schemas/context/segmentidentity.example.1.json

@@ -0,0 +1,6 @@
+{
+  "@id": "https://data.adobe.io/entities/identity/id123",
+  "xdm:namespace": {
+    "xdm:code": "AA12345"
+  }
+}

schemas/context/segmentidentity.schema.json

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+{
+  "meta:license": [
+    "Copyright 2018 Adobe Systems Incorporated. All rights reserved.",
+    "This work is licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) license",
+    "you may not use this file except in compliance with the License. You may obtain a copy",
+    "of the License at https://creativecommons.org/licenses/by/4.0/"
+  ],
+  "$id": "https://ns.adobe.com/xdm/context/segmentidentity",
+  "$schema": "http://json-schema.org/draft-06/schema#",
+  "title": "Identity",
+  "type": "object",
+  "description":
+    "SegmentIdentity is used to clearly distinguish segments. SegmentIdentity is established by an segmentidentity provider, which itself is referenced in the `namespace` attribute. Within each `namespace`, the segmentidentity is unique.",
+  "definitions": {
+    "identity": {
+      "properties": {
+        "@id": {
+          "title": "Identifier",
+          "type": "string",
+          "format": "uri",
+          "description": "Identity of the Segment in the related namespace."
+        },
+        "xdm:namespace": {
+          "title": "Namespace",
+          "$ref": "https://ns.adobe.com/xdm/context/namespace",
+          "description":
+            "The namespace associated with the `xid` attribute."
+        },
+        "xdm:xid": {
+          "title": "Experience Identifier",
+          "type": "string",
+          "description":
+            "When present, this value represents a cross-namespace identifier that is unique across all namespace-scoped identifiers in all namespaces."
+        }
+      }
+    }
+  },
+  "allOf": [
+    {
+      "$ref": "#/definitions/segmentidentity"
+    }
+  ],
+  "meta:status": "experimental"
+}