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.

cmd/iam.go

@@ -681,7 +681,7 @@ func (sys *IAMSys) SetTempUser(ctx context.Context, accessKey string, cred auth.
 		return errServerNotInitialized
 	}
 
-	if globalPolicyOPA != nil {
+	if globalAuthZPlugin != nil {
 		// If OPA is set, we do not need to set a policy mapping.
 		policyName = ""
 	}
@@ -1693,8 +1693,8 @@ func (sys *IAMSys) GetCombinedPolicy(policies ...string) iampolicy.Policy {
 // IsAllowed - checks given policy args is allowed to continue the Rest API.
 func (sys *IAMSys) IsAllowed(args iampolicy.Args) bool {
 	// If opa is configured, use OPA always.
-	if globalPolicyOPA != nil {
-		ok, err := globalPolicyOPA.IsAllowed(args)
+	if globalAuthZPlugin != nil {
+		ok, err := globalAuthZPlugin.IsAllowed(args)
 		if err != nil {
 			logger.LogIf(GlobalContext, err)
 		}