Consolidate CDK custom-resources deps into root package.json

Merge custom-resources/package.json dependencies into the root
package.json and remove the separate package file. This simplifies
the Quick Start from two npm install commands to one.

The custom resource Lambda handlers use NodejsFunction with esbuild,
which resolves imports via standard Node module resolution — walking
up to the root node_modules works correctly.

Dependencies moved to root:
- @aws-crypto/sha256-js (bundled into UI Init Lambda)
- @aws-sdk/credential-provider-node (external, Lambda runtime)
- @aws-sdk/signature-v4 (bundled into UI Init Lambda)
- @smithy/protocol-http (bundled into UI Init Lambda)

Dependencies dropped (excluded via externalModules and provided by
Lambda Node.js 22 runtime):
- @aws-sdk/client-opensearch
- @aws-sdk/client-secrets-manager

Tested: cdk synth + full cdk deploy --all with all 4 custom resource
Lambdas (FGAC mapping, DQS datasource, OpenSearch App, UI Init)
completing successfully.

Author: kylehounslow

aws/cdk/README.md

@@ -14,7 +14,6 @@ Deploy the Observability Stack to AWS managed services using CDK. This creates t
 ```bash
 cd aws/cdk
 npm install
-cd custom-resources && npm install && cd ..
 cdk deploy --all
 ```