refactor(enmeshed): move serialization for creating IdentityAttributes to Attribute::Identity

The former implementation of `Attibute.to_json` was very specific to IdentityAttributes.
It actually only worked for simple IdentityAttributes and would need adaptations for complex
IdentityAttributes.

Part of XI-6523

Author: nenock

lib/enmeshed/attribute.rb

@@ -32,21 +32,14 @@ def self.parse(content)
       desired_klass.new(**attributes.compact)
     end
 
+    # Serialize all available attributes of an Attribute object.
+    # E.g. for sharing it when making the API request to create a relationship template.
+    #
+    # @return [Hash]
     def to_h
       default_attributes.deep_merge(additional_attributes)
     end
 
-    def to_json(*)
-      {
-        content: {
-          value: {
-            '@type': @type,
-            value: @value,
-          },
-        },
-      }.to_json(*)
-    end
-
     def id
       @id ||= persistent_id
     end

lib/enmeshed/attribute/identity.rb

@@ -2,6 +2,21 @@
 
 module Enmeshed
   class Attribute::Identity < Attribute
+    # Serialize the Identity object to fit the content requirements when creating
+    # an IdentityAttribute via the Connector API (available under Connector::CONNECTOR_URL)
+    #
+    # @return [String] JSON
+    def to_json(*)
+      {
+        content: {
+          value: {
+            '@type': @type,
+            value: @value,
+          },
+        },
+      }.to_json(*)
+    end
+
     private
 
     def additional_attributes