feat: custom waf and other fixes

[leonchabbey]

Description

Adds the possibility to use a custom waf (externally created) and also fixes some minor issues.

Type of changes

• [x] Refactoring (non-breaking change)
• [x] 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)

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Sensitive Information Declaration

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

[Tomdango]

Thanks for the PR! Looks like this closes https://github.com/nhs-england-tools/terraform-aws-opennext/pull/7.