refactor: create common package for holding STUN input (#631)

We want stunreachability to use the same STUN servers used by
snowflake, so let's start by making a common package holding the
servers. Let's also use this new package in Snowflake.

We're currently not using this package in stunreachability, but
I am going to apply this as a subsequent diff.

Reference issue: https://github.com/ooni/probe/issues/1814. This
issue is a bit complex to address in a single PR, so we are going
to proceed incremntally.

This diff was extracted from https://github.com/ooni/probe-cli/pull/539.

internal/stuninput/stuninput_test.go

@@ -0,0 +1,31 @@
+package stuninput
+
+import "testing"
+
+func TestAsSnowflakeInput(t *testing.T) {
+	outputs := AsSnowflakeInput()
+	if len(outputs) != len(inputs) {
+		t.Fatal("unexpected number of entries")
+	}
+	for idx := 0; idx < len(inputs); idx++ {
+		output := outputs[idx]
+		input := "stun:" + inputs[idx]
+		if input != output {
+			t.Fatal("mismatch")
+		}
+	}
+}
+
+func TestAsStunReachabilityInput(t *testing.T) {
+	outputs := AsnStunReachabilityInput()
+	if len(outputs) != len(inputs) {
+		t.Fatal("unexpected number of entries")
+	}
+	for idx := 0; idx < len(inputs); idx++ {
+		output := outputs[idx]
+		input := "stun://" + inputs[idx]
+		if input != output {
+			t.Fatal("mismatch")
+		}
+	}
+}