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/auth-handler.go

@@ -217,8 +217,8 @@ func getClaimsFromTokenWithSecret(token, secret string) (map[string]interface{},
 		}
 	}
 
-	// If OPA is set, return without any further checks.
-	if globalPolicyOPA != nil {
+	// If AuthZPlugin is set, return without any further checks.
+	if globalAuthZPlugin != nil {
 		return claims.Map(), nil
 	}