add requestTimeEpoch to aws lambda types (#22941)

Add the missing requestTimeEpoch in the APIGatewayEventRequestContext interface.

$context.requestTimeEpoch is documented in https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html#context-variable-reference

types/aws-lambda/index.d.ts

@@ -39,6 +39,7 @@ interface APIGatewayEventRequestContext {
     },
     stage: string;
     requestId: string;
+    requestTimeEpoch: number;
     resourceId: string;
     resourcePath: string;
 }