CCM-7908 adding mechanism for TFVAR based secret

[aidenvaines-bjss]

Description

Adds a mechanism by which the authorization headers for the three microservices can be set consistently so that communication between auth, templates and the gateway can be centrally managed per

TF_VAR_amplify_basic_auth_secret is set on the internal repo already, merging this will cause access to be limited while the CDN, IAM-Webauth, and Web-Templates are deployed and pick this up

Context

TDA-2024-03

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.