Add support for custom data fields which extend the Pledge object.
When defined, these custom field(s) should be available for input in create / edit Pledge from the Pledge tab. (We will not expose them for self-service pledges at this time.)
When defined, they should be output for:
- View Pledge page
- AcknowledgeMessage.tpl (acknowledgement email)
If custom field is searchable - then you it's included in Find Pledges search form and Advanced Search - Pledges pane.