[decimal] Implement add for BigDecimal

benches/bigdecimal/bench.mojo

@@ -0,0 +1,25 @@
+from bench_bigdecimal_add import main as bench_add
+
+
+fn main() raises:
+    print(
+        """
+=========================================
+This is the BigInt Benchmarks
+=========================================
+add:         Add
+all:         Run all benchmarks
+q:           Exit
+=========================================
+"""
+    )
+    var command = input("Type name of bench you want to run: ")
+    if command == "add":
+        bench_add()
+    elif command == "all":
+        bench_add()
+    elif command == "q":
+        return
+    else:
+        print("Invalid input")
+        main()