Add support for Access Management Plugin (#14875)

- This change renames the OPA integration as Access Management Plugin - there is
nothing specific to OPA in the integration, it is just a webhook.

- OPA configuration is automatically migrated to Access Management Plugin and
OPA specific configuration is marked as deprecated.

- OPA doc is updated and moved.

internal/config/policy/plugin/help.go

@@ -0,0 +1,49 @@
+// Copyright (c) 2015-2022 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// 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 plugin
+
+import "github.com/minio/minio/internal/config"
+
+// Help template for Access Management Plugin policy feature.
+var (
+	defaultHelpPostfix = func(key string) string {
+		return config.DefaultHelpPostfix(DefaultKVS, key)
+	}
+
+	Help = config.HelpKVS{
+		config.HelpKV{
+			Key:         URL,
+			Description: `plugin hook endpoint (HTTP(S)) e.g. "http://localhost:8181/v1/data/httpapi/authz/allow"` + defaultHelpPostfix(URL),
+			Type:        "url",
+			Sensitive:   true,
+		},
+		config.HelpKV{
+			Key:         AuthToken,
+			Description: "authorization token for plugin hook endpoint" + defaultHelpPostfix(AuthToken),
+			Optional:    true,
+			Type:        "string",
+			Sensitive:   true,
+		},
+		config.HelpKV{
+			Key:         config.Comment,
+			Description: config.DefaultComment,
+			Optional:    true,
+			Type:        "sentence",
+		},
+	}
+)