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/opa/help.go

@@ -28,13 +28,13 @@ var (
 	Help = config.HelpKVS{
 		config.HelpKV{
 			Key:         URL,
-			Description: `OPA HTTP(s) endpoint e.g. "http://localhost:8181/v1/data/httpapi/authz/allow"` + defaultHelpPostfix(URL),
+			Description: `[DEPRECATED] OPA HTTP(s) endpoint e.g. "http://localhost:8181/v1/data/httpapi/authz/allow"` + defaultHelpPostfix(URL),
 			Type:        "url",
 			Sensitive:   true,
 		},
 		config.HelpKV{
 			Key:         AuthToken,
-			Description: "authorization token for OPA endpoint" + defaultHelpPostfix(AuthToken),
+			Description: "[DEPRECATED] authorization token for OPA endpoint" + defaultHelpPostfix(AuthToken),
 			Optional:    true,
 			Type:        "string",
 			Sensitive:   true,