rs: add «2971. Find Polygon With the Largest Perimeter»

Signed-off-by: Matej Focko <mfocko@redhat.com>

rs/find-polygon-with-the-largest-perimeter.rs

@@ -0,0 +1,20 @@
+impl Solution {
+    pub fn largest_perimeter(mut nums: Vec<i32>) -> i64 {
+        nums.sort();
+
+        let mut sum: i64 = 0;
+        let mut max_perimeter: i64 = -1;
+
+        for x in &nums {
+            let x = i64::from(*x);
+
+            if x < sum {
+                max_perimeter = x + sum;
+            }
+
+            sum += x;
+        }
+
+        max_perimeter
+    }
+}