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/iam/validator/validators_test.go

@@ -0,0 +1,64 @@
+/*
+ * 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 validator
+
+import (
+	"testing"
+)
+
+type errorValidator struct{}
+
+func (e errorValidator) Validate(token, dsecs string) (map[string]interface{}, error) {
+	return nil, ErrTokenExpired
+}
+
+func (e errorValidator) ID() ID {
+	return "err"
+}
+
+func TestValidators(t *testing.T) {
+	vrs := NewValidators()
+	if err := vrs.Add(&errorValidator{}); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := vrs.Add(&errorValidator{}); err == nil {
+		t.Fatal("Unexpected should return error for double inserts")
+	}
+
+	if _, err := vrs.Get("unknown"); err == nil {
+		t.Fatal("Unexpected should return error for unknown validators")
+	}
+
+	v, err := vrs.Get("err")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err = v.Validate("", ""); err != ErrTokenExpired {
+		t.Fatalf("Expected error %s, got %s", ErrTokenExpired, err)
+	}
+
+	vids := vrs.List()
+	if len(vids) == 0 || len(vids) > 1 {
+		t.Fatalf("Unexpected number of vids %v", vids)
+	}
+
+	if vids[0] != "err" {
+		t.Fatalf("Unexpected vid %v", vids[0])
+	}
+}