Added an OpenAPI validation target and license headers.

Makefile

@@ -107,6 +107,16 @@ oas-download: ## Downloads the latest App Store Connect OpenAPI specification do
 oas-patch: # Patches the OpenAPI specification document with the patches in Scripts/patch-oas.jq
 	@jq --from-file Scripts/patch-oas.jq "$(OAS_DOWNLOAD_OUTPUT)" > "$(OAS_DOWNLOAD_OUTPUT).tmp"
 	@mv "$(OAS_DOWNLOAD_OUTPUT).tmp" "$(OAS_DOWNLOAD_OUTPUT)"
+	@$(MAKE) --no-print-directory oas-validate
+
+oas-validate: ## Validates the OpenAPI specification document
+	@swift run \
+		--configuration release \
+		--package-path "$(OAS_GENERATE_TOOLS)" \
+		swift-openapi-generator generate \
+		--config "$(OAS_GENERATE_CONFIG)" \
+		--dry-run \
+		"$(OAS_DOWNLOAD_OUTPUT)"
 
 oas-generate: ## Generates the library source code from the OpenAPI specification document
 	@mkdir -p "$(OAS_GENERATE_OUTPUT)"