new event source variable

[masl2]

Description

Introduces a new terraform variable to map to the source field of supplier api produced events.
This changes from deriving it from the terraform scaffold variables.

Implemented to allow greater control of this value and ensure it matches with the consuming domains' filters

Core and Supplier API source expectations did not match causing all supplier status updates to be filtered.
A hotfix was applied to PROD, this retrofits this fix to the trunk

Pairs with https://github.com/NHSDigital/nhs-notify-internal/pull/586

• 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
• If I have used the 'skip-trivy-package' label I have done so responsibly and in the knowledge that this is being fixed as part of a separate ticket/PR.

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