integration/raindrop: initial draft implementation

[kejadlen]

Do you follow the guidelines?

• [x] I have tested my changes
• [x] There is no breaking changes
• [x] I really tested my changes and there is no regression
• [x] Ideally, my commit messages use the same convention as the Go project: https://go.dev/doc/contribute#commit_messages
• [x] I read this document: https://miniflux.app/faq.html#pull-request