Add meta_data for items (#1576)

* PoC for ItemMetaProperties/Values

* Missing is_valid

* ItemMetaProperties/Values in editable via API, cloneable

* Tests

* Add Docs

* Fix import order

* Fix another import sorting...

* Typeahead for ItemMetaValues

* Test for editing event-objects

* Fix typeahead permission checks

* Further access restriction

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

doc/api/resources/items.rst

@@ -114,6 +114,7 @@ bundles                               list of objects            Definition of b
 └ designated_price                    money (string)             Designated price of the bundled product. This will be
                                                                  used to split the price of the base item e.g. for mixed
                                                                  taxation. This is not added to the price.
+meta_data                             object                     Values set for event-specific meta data parameters.
 ===================================== ========================== =======================================================
 
 .. versionchanged:: 2.7
@@ -154,6 +155,10 @@ bundles                               list of objects            Definition of b
 
    The ``show_quota_left``, ``allow_waitinglist``, and ``hidden_if_available`` attributes have been added.
 
+.. versionchanged:: 3.6
+
+   The attribute ``meta_data`` has been added.
+
 Notes
 -----
 
@@ -208,6 +213,7 @@ Endpoints
             "tax_rule": 1,
             "admission": false,
             "issue_giftcard": false,
+            "meta_data": {},
             "position": 0,
             "picture": null,
             "available_from": null,
@@ -303,6 +309,7 @@ Endpoints
         "tax_rule": 1,
         "admission": false,
         "issue_giftcard": false,
+        "meta_data": {},
         "position": 0,
         "picture": null,
         "available_from": null,
@@ -379,6 +386,7 @@ Endpoints
         "tax_rule": 1,
         "admission": false,
         "issue_giftcard": false,
+        "meta_data": {},
         "position": 0,
         "picture": null,
         "available_from": null,
@@ -442,6 +450,7 @@ Endpoints
         "tax_rule": 1,
         "admission": false,
         "issue_giftcard": false,
+        "meta_data": {},
         "position": 0,
         "picture": null,
         "available_from": null,
@@ -537,6 +546,7 @@ Endpoints
         "tax_rule": 1,
         "admission": false,
         "issue_giftcard": false,
+        "meta_data": {},
         "position": 0,
         "picture": null,
         "available_from": null,