go: add «1922. Count Good Numbers»

URL:	https://leetcode.com/problems/count-good-numbers/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/count-good-numbers.go

@@ -0,0 +1,22 @@
+package main
+
+const (
+	MOD int64 = 1000000007
+)
+
+func countGoodNumbers(n int64) int {
+	quickmul := func(x, y int64) int64 {
+		res := int64(1)
+		for y > 0 {
+			if y%2 == 1 {
+				res = (res * x) % MOD
+			}
+			x = (x * x) % MOD
+			y /= 2
+		}
+
+		return res
+	}
+
+	return int((quickmul(5, (n+1)/2) * quickmul(4, n/2)) % MOD)
+}