Add FriendshipManager.sent_requests()

The `FriendshipManager` has convenience functions with caching to easily get friendship requests sent *to* a user (`requests()`, `unread_requests()`, ...). There is no function for retrieving a list of all friendship requests a user has sent (*from* user). I previously proposed adding `FriendshipManager.sent_requests()` in another pull request and just create this issue to setup a separate pull request for that now.

- [x] add `FriendshipManager.sent_requests()`
- [x] write tests for `FriendshipManager.sent_requests()`