Introduce STS client grants API and OPA policy integration (#6168)

This PR introduces two new features

- AWS STS compatible STS API named AssumeRoleWithClientGrants

```
POST /?Action=AssumeRoleWithClientGrants&Token=<jwt>
```

This API endpoint returns temporary access credentials, access
tokens signature types supported by this API

  - RSA keys
  - ECDSA keys

Fetches the required public key from the JWKS endpoints, provides
them as rsa or ecdsa public keys.

- External policy engine support, in this case OPA policy engine

- Credentials are stored on disks

pkg/madmin/encrypt.go

@@ -0,0 +1,68 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package madmin
+
+import (
+	"bytes"
+	"crypto/rand"
+	"io"
+	"io/ioutil"
+
+	"github.com/minio/sio"
+	"golang.org/x/crypto/argon2"
+)
+
+// EncryptData - encrypts server config data.
+func EncryptData(password string, data []byte) ([]byte, error) {
+	salt := make([]byte, 32)
+	if _, err := io.ReadFull(rand.Reader, salt); err != nil {
+		return nil, err
+	}
+
+	// derive an encryption key from the master key and the nonce
+	var key [32]byte
+	copy(key[:], argon2.IDKey([]byte(password), salt, 1, 64*1024, 4, 32))
+
+	encrypted, err := sio.EncryptReader(bytes.NewReader(data), sio.Config{
+		Key: key[:]},
+	)
+	if err != nil {
+		return nil, err
+	}
+	edata, err := ioutil.ReadAll(encrypted)
+	return append(salt, edata...), err
+}
+
+// DecryptData - decrypts server config data.
+func DecryptData(password string, data io.Reader) ([]byte, error) {
+	salt := make([]byte, 32)
+	if _, err := io.ReadFull(data, salt); err != nil {
+		return nil, err
+	}
+	// derive an encryption key from the master key and the nonce
+	var key [32]byte
+	copy(key[:], argon2.IDKey([]byte(password), salt, 1, 64*1024, 4, 32))
+
+	decrypted, err := sio.DecryptReader(data, sio.Config{
+		Key: key[:]},
+	)
+	if err != nil {
+		return nil, err
+	}
+	return ioutil.ReadAll(decrypted)
+}