NATS TLS specify CA and client TLS authentication (#8389)

- added ability to specify CA for self-signed certificates
- added option to authenticate using client certificates
- added unit tests for nats connections

pkg/event/target/testdata/nats_tls_client_cert.conf

@@ -0,0 +1,18 @@
+port: 14226
+net: localhost
+
+tls {
+    cert_file:  "./testdata/certs/nats_server_cert.pem"
+    key_file:   "./testdata/certs/nats_server_key.pem"
+    ca_file:   "./testdata/certs/root_ca_cert.pem"
+    verify_and_map: true
+}
+authorization {
+    ADMIN = {
+        publish = ">"
+        subscribe = ">"
+    }
+    users = [
+        {user: "CN=localhost,OU=Client,O=Minio,C=CA", permissions: $ADMIN}
+    ]
+}