CCM-7283: Use self-container Cognito User Pool as a temporary auth solution

[chris-elliott-nhsd]

Description

This is a PR to use the user pool that is created as part of the Amplify backend for auth. This is a strictly temporary solution as we would like to move in the medium term to using the separate auth microservice, however this cannot be done until we have done some additional work to move away from Amplify data which does not support external auth providers.

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.