feat(aws-lambda): added ability to configure extension timeout

- I saw a couple of timeouts while testing

> The Instana Lambda extension Heartbeat request did not succeed. Falling back to talking to the Instana back end directly. Error: The Lambda extension Heartbeat request timed out.

- added INSTANA_LAMBDA_EXTENSION_TIMEOUT_IN_MS
- increased the default timeout to 500ms

packages/serverless/src/backend_connector.js

@@ -20,7 +20,9 @@ let useLambdaExtension = false;
 
 const timeoutEnvVar = 'INSTANA_TIMEOUT';
 let defaultTimeout = 500;
-const layerExtensionTimeout = 300;
+const layerExtensionTimeout = process.env.INSTANA_LAMBDA_EXTENSION_TIMEOUT_IN_MS
+  ? Number(process.env.INSTANA_LAMBDA_EXTENSION_TIMEOUT_IN_MS)
+  : 500;
 let backendTimeout = defaultTimeout;
 
 const proxyEnvVar = 'INSTANA_ENDPOINT_PROXY';