Mailing API - Expose option list for header_id and footer_id

[totten]

Overview

In APIv3, the actions getoptions and getfields are able to list options for most fields. This expands that function to more fields.

Before

The API does not return the option list for Mailing.header_id or Mailing.footer_id.

$ cv api Mailing.getoptions field=header_id
{
    "is_error": 1,
    "error_message": "The field 'header_id' has no associated option list."
}

After

The API does return the option list for Mailing.header_id or Mailing.footer_id.

$ cv api Mailing.getoptions field=header_id
{
    "is_error": 0,
    "version": 3,
    "count": 1,
    "id": 1,
    "values": {
        "1": "Mailing Header"
    }
}

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[seamuslee001]

Change looks good to me merging