Add Comment API Namespace

[matthewruzzi]

Is your feature request related to a problem? Please describe.
Using the Comment API Namespace in FeedKit

Describe the solution you'd like
It would be nice if FeedKit supported the Comment API Namespace for RSS.
https://www.rssboard.org/comment-api

Describe alternatives you've considered
If FeedKit supported accessing tags from arbitrary namespaces that don't have hard coded support that would also work.
#197

Additional context
https://feedsmith.dev/reference/namespaces/wfw