feat: Decouple `extensionLayerVersion` from `addLayers`

[samchungy]

What does this PR do?

Resolves https://github.com/DataDog/datadog-cdk-constructs/issues/201

• Allows the extension layer to be added regardless of the addLayers property.

Motivation

I would like to instrument my lambda with the node libraries without layers but the extension library is required to be added as a layer.

At the moment I use an ugly workaround to allow for this behaviour.

https://github.com/seek-oss/skuba/blob/ca287dfa624f1739fbb349bc22dfde3be9b09919/template/lambda-sqs-worker-cdk/infra/appStack.ts#L130-L139

Testing Guidelines

• Unit Tests

Additional Notes

Types of Changes

• [ ] Bug fix
• [x] New feature
• [x] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Arguably this is a minor breaking change if the user was relying on addLayers to prevent layers from being added whilst specifying the extensionLayerVersion but I would hope that this is uncommon.

Check all that apply

• [x] This PR's description is comprehensive
• [x] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [x] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[samchungy]

Heyo @neko-dd and/or @astuyve or @duncanista could I trouble you for a merge/release or more feedback if this approach is particularly troubling? Cheers 😄

[neko-dd]

Docs doesn't merge in this repo - I'll check in tomorrow to see if someone else has merged this, and if not I'll try to escalate for you. Thanks!

[lym953]

Confirmed with @purple4reina and @TophrC-dd that this PR is good. Merging.

[lym953]

Merged and released.

[samchungy]

Appreciate it, thanks!