PPHA-685: Pen testing hardening changes

[mrlockstar]

Description

Added allow_nested_items_to_be_public, this allows nested that are contained inside the blob storage to become public if enabled, please note, this ONLY works if the storage account is public.

Also improve the description for variable "shared_access_key_enabled" as disabling this feature requires config changes in the root module.

https://dev.azure.com/nhse-dtos/lung-cancer-screening/_build/results?buildId=39378&view=results

Context

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.