Add separate counts for replies, messages and mentions to unread count API endpoint

### Requirements - [x] Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support or the [matrix chat](https://matrix.to/#/#lemmy:matrix.org). - [x] Did you check to see if this issue already exists? - [x] Is this only a feature request? Do not put multiple feature requests in one issue. - [x] Is this a backend issue? Use the [lemmy-ui](https://github.com/LemmyNet/lemmy-ui) repo for UI / frontend issues. - [x] Do you agree to follow the rules in our [Code of Conduct](https://join-lemmy.org/docs/code_of_conduct.html)? ### Is your proposal related to a problem? The old API returned separate count numbers for unread replies, mentions and private messages. The new API only returns a combined number for all 3. Can the new API be changed to also include the separate unread numbers? Separate numbers are useful as I have some UI that displays the user notifications all in one spot and I separate the categories by "tabs" with a unread count for each tab. Without separate counts I can't display a unread count for each individual tab. ### Describe the solution you'd like. Update the unread count API to include individual fields for unread replies, mentions and private messages. ### Describe alternatives you've considered. I can iterate the list of unreads manually but this can spam the server. I can use this as a workaround though if this feature can't be implemented. ### Additional context _No response_