feat(lib): answer UserInfoRequest with our slots and queue

Opening a user's profile in SoulseekQt or Nicotine+ asks that peer
directly (peer code 15), and a peer that never answers shows an endless
spinner. The reply (code 16) carries no description or picture, the
configured upload slot count, how many uploads are waiting, and whether a
slot is free right now, which is what the reference clients send.

soulseek-rs-lib/src/client/mod.rs

@@ -292,6 +292,10 @@ pub enum ClientOperation {
     ShareListRequested {
         requester_key: String,
     },
+    /// A peer asked what we say about ourselves; send a UserInfoResponse.
+    UserInfoRequested {
+        requester_key: String,
+    },
     /// A peer asked for one folder of our shares; send a FolderContentsResponse.
     FolderContentsRequested {
         requester_key: String,