feat(lib): answer FolderContentsRequest with the folder's listing

"Download folder" in SoulseekQt and Nicotine+ asks the sharer for that
one folder (peer code 36) instead of the whole share, and a sharer that
never answers leaves the folder download hanging. The requested folder
now comes back as a FolderContentsResponse (code 37), which carries a
directory listing in the same form a browse does, so the two share one
codec. The browse and folder replies also share one reply helper.

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

@@ -292,6 +292,12 @@ pub enum ClientOperation {
     ShareListRequested {
         requester_key: String,
     },
+    /// A peer asked for one folder of our shares; send a FolderContentsResponse.
+    FolderContentsRequested {
+        requester_key: String,
+        token: u32,
+        folder: String,
+    },
     /// A peer we are browsing returned their shared-file listing.
     BrowseResult {
         username: String,