idp integration for mcs (#75)

This PR adds support for oidc in  mcs, to enable idp
authentication you need to pass the following environment variables and
restart mcs.

```
MCS_IDP_URL=""
MCS_IDP_CLIENT_ID=""
MCS_IDP_SECRET=""
MCS_IDP_CALLBACK=""
```

pkg/auth/utils/utils_test.go

@@ -0,0 +1,46 @@
+// This file is part of MinIO Console Server
+// Copyright (c) 2020 MinIO, Inc.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package utils
+
+import (
+	"crypto/sha1"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/crypto/pbkdf2"
+)
+
+func TestRandomCharString(t *testing.T) {
+	funcAssert := assert.New(t)
+	// Test-1 : RandomCharString() should return string with expected length
+	length := 32
+	token := RandomCharString(length)
+	funcAssert.Equal(length, len(token))
+	// Test-2 : RandomCharString() should output random string, new generated string should not be equal to the previous one
+	newToken := RandomCharString(length)
+	funcAssert.NotEqual(token, newToken)
+}
+
+func TestComputeHmac256(t *testing.T) {
+	funcAssert := assert.New(t)
+	// Test-1 : ComputeHmac256() should return the right Hmac256 string based on a derived key
+	var derivedKey = pbkdf2.Key([]byte("secret"), []byte("salt"), 4096, 32, sha1.New)
+	var message = "hello world"
+	var expectedHmac = "5r32q7W+0hcBnqzQwJJUDzVGoVivXGSodTcHSqG/9Q8="
+	hmac := ComputeHmac256(message, derivedKey)
+	funcAssert.Equal(hmac, expectedHmac)
+}