CustomFieldAdminPlugin: Handle possible keys mismatch with order admin. Closes #11540.

git-svn-id: http://trac-hacks.org/svn/customfieldadminplugin/0.11@13732 7322e99d-02ea-0310-aa39-e9a107903beb
CustomFieldAdminPlugin: Test for #10082.

git-svn-id: http://trac-hacks.org/svn/customfieldadminplugin/0.11@13733 7322e99d-02ea-0310-aa39-e9a107903beb
Added `ICustomFieldTypeProvider` interface which is allowed plugins to provide additional types of custom field