feat: add some RSS API methods

[Yejia995]

Added the following RSS API methods:

• [x] Add folder
• [x] Add feed
• [x] Remove item
• [x] Move item
• [ ] Get all items
• [x] Mark as read
• [x] Refresh item
• [ ] Set auto-downloading rule
• [x] Rename auto-downloading rule
• [x] Remove auto-downloading rule
• [ ] Get all auto-downloading rules
• [ ] Get all articles matching a rule

This pull request does not include any new struct, only relatively simple operations. All functions should have been tested, but if there are any issues please let me know.

Thanks a lot for your work!