Improve the File Overwrite Handling for the File Outbound Channel Adapter

We recently added support for file appends with INT-2470. In order to improve the behavior further, we will remove the "append" attribute and replace it with a new enumeration called mode, providing the following options:

• Replace - The Default Setting
• Append - Same functionality as INT-2470
• Fail - Raise an exception when the file already exists (new)
• Ignore - Silently ignore the message payload file if the destination file already exists (new)