Enhance policy handling to support SSE and WORM (#5790)

- remove old bucket policy handling
- add new policy handling
- add new policy handling unit tests

This patch brings support to bucket policy to have more control not
limiting to anonymous.  Bucket owner controls to allow/deny any rest
API.

For example server side encryption can be controlled by allowing
PUT/GET objects with encryptions including bucket owner.

pkg/policy/effect.go

@@ -0,0 +1,78 @@
+/*
+ * 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 policy
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// Effect - policy statement effect Allow or Deny.
+type Effect string
+
+const (
+	// Allow - allow effect.
+	Allow Effect = "Allow"
+
+	// Deny - deny effect.
+	Deny = "Deny"
+)
+
+// IsAllowed - returns if given check is allowed or not.
+func (effect Effect) IsAllowed(b bool) bool {
+	if effect == Allow {
+		return b
+	}
+
+	return !b
+}
+
+// IsValid - checks if Effect is valid or not
+func (effect Effect) IsValid() bool {
+	switch effect {
+	case Allow, Deny:
+		return true
+	}
+
+	return false
+}
+
+// MarshalJSON - encodes Effect to JSON data.
+func (effect Effect) MarshalJSON() ([]byte, error) {
+	if !effect.IsValid() {
+		return nil, fmt.Errorf("invalid effect '%v'", effect)
+	}
+
+	return json.Marshal(string(effect))
+}
+
+// UnmarshalJSON - decodes JSON data to Effect.
+func (effect *Effect) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+
+	e := Effect(s)
+	if !e.IsValid() {
+		return fmt.Errorf("invalid effect '%v'", s)
+	}
+
+	*effect = e
+
+	return nil
+}