Register postgre driver in pkg/event/target (#6689)

Commit 5c13765168 removed postgre registration triggerd
by the automatic gofmt command but it was the only where pg is registered. This commit
fixes behavior and adds unit tests to check whether postgre & sql are registered or not.

vendor/github.com/lib/pq/ssl_permissions.go

@@ -0,0 +1,20 @@
+// +build !windows
+
+package pq
+
+import "os"
+
+// sslKeyPermissions checks the permissions on user-supplied ssl key files.
+// The key file should have very little access.
+//
+// libpq does not check key file permissions on Windows.
+func sslKeyPermissions(sslkey string) error {
+	info, err := os.Stat(sslkey)
+	if err != nil {
+		return err
+	}
+	if info.Mode().Perm()&0077 != 0 {
+		return ErrSSLKeyHasWorldPermissions
+	}
+	return nil
+}