Add Datadog example for streaming mode

[lym953]

Issue #, if available:
#506 (comment)

Description of changes:

1. Add an example app in response_stream mode, instrumented with Datadog
2. For existing Datadog examples:
3. Update extension layer version
4. Add a param architecture with a comment. I got an error during Lambda execution without this.
5. Some auto formatting

Notes:

1. This example is mostly copied from the existing example examples/datadog/expressjs by @alexgallotta
2. The Datadog APM support for streaming mode was added by @duncanista fix(proxy): stream requests DataDog/datadog-lambda-extension#771

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[bnusunny]

LGTM