utils: small_vector: expose internal_capacity()

So we can use it for defining other small_vector
deriving their internal capacity from another small_vector
type.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

utils/small_vector.hh

@@ -251,6 +251,10 @@ public:
         }
     }
 
+    static constexpr size_t internal_capacity() noexcept {
+        return N;
+    }
+
     size_t external_memory_usage() const {
         if (uses_internal_storage()) {
             return 0;