Support for optionally returning plain-text release notes for Sparkle API.

[GetToSet]

Describe the solution you'd like

Showing HTML release notes for Mac Catalyst apps is not supported by Sparkle, release notes will be displayed as plain text with the following log messages:

Error: Showing HTML release notes for Catalyst apps is not supported. The release notes will be interpreted as plain text. Please serve a plain-text (.txt) release notes file. If you are using a element then please specify the sparkle:format="plain-text" attribute in that element.

The AppCenter API for Sparkle feed should accept an option to return plain-text description.

Describe alternatives you've considered

Currently it seems there's no workaround for this issue.

Additional context

Add any other context or screenshots about the feature request here.

• Catalyst Release Notes Support sparkle-project/Sparkle#1482