feat: allow solution to config internal system user

[sydefz]

*This PR enables customisation on druid internal system user, once the cluster is provisioned owner can change the internal system user credentials in secret manager to customise it. The solution now pulls the system user secret and set username/password as environment variables, the variable is then handled in render_druid_config.py and check_druid_status.py script so it applies to the entire cluster.

We also updated render_utils.py to correctly handle dict properties. For example previously "druid.auth.authenticator.allowAll.initialInternalClientPassword": {"type": "environment", "variable": "DRUID_INTERNAL_CLIENT_PASSWORD"} will be rendered as string into the file (properties[key] = str(value)) which makes the final file has {'type': 'environment', 'variable': 'DRUID_INTERNAL_CLIENT_PASSWORD'} property value, this will cause druid to confuse about where to get the value from INFO [main] org.apache.druid.guice.JsonConfigurator - Unable to parse value of property [druid.auth.authenticator.allowAll.initialInternalClientPassword] as a json object, using as is.*

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[sydefz]

For testing we tested the following flow to ensure the customisation is working.

• Spin up a new cluster
• Create a new user (eg.aws_druid_system ) for internal communication using Druid API
• Modify the secret in AWS secrets manager to use the newly created user. eg. secret payload = {"username": "aws_druid_system", "password": "xxxxxx"}
• Redeploy the solution (npm run cdk deploy) to make the cluster use the new user aws_druid_system for internal communication

[van-vothanh]

Thank you for your contribution @sydefz
This change has now been verified against our pipeline.