dht/ring_position: add ring_position_view::weight()

This will be useful for the translation of ring positions
to BTI encoding.
We will use it in a later commit.

dht/ring_position.hh

@@ -251,6 +251,7 @@ public:
 
     const dht::token& token() const noexcept { return *_token; }
     const partition_key* key() const { return _key; }
+    int weight() const { return _weight; }
 
     // Only when key() == nullptr
     token_bound get_token_bound() const { return token_bound(_weight); }